When Bobby is captured by the Leviathans and taken to Dick Roman's office in 7.09 How to Win Friends and Influence Monsters, he gets a chance to examine the files in Roman's briefcase which relate to the Leviathan's grand plan. There appear to be a number of schematics, and maps relating to the location of infrastructure such as dams and power plants. As he examines a map, with red markers in Wisconsin and Ohio, he seems to have an epiphany as to what the Leviathans intend to do.
As Bobby is escaping from the Leviathans, he is shot in the head by Dick as he gets in the van with Sam and Dean. As he lapses into a coma, Bobby's subconscious is insistent that he remember something of importance that he must tell the boys. He writes down the numbers 454895. As his brain starts shutting down, Bobby struggles to confront his worst memory in order to regain consciousness. He manages to long enough to write the numbers on Sam's hand, although he only writes the first five numbers. With a final affectionate "Idjits", Bobby dies.
After Bobby's death, Dean tries to interpret the numbers, and calls on Frank Devereaux to help. It is Frank who starts trying to find meanings for a 6 digit number. Using 454893, one digit different to the number Bobby wrote down, he generates the coordinates latitude (45.4) and longitude (-89.3) for a field in Gleason Wisconsin. The land has recently been purchased by Wellman Inc, a subsidiary of Richard Roman Enterprises. Surveillance reveals that an employee of Dick Roman, Amanda Willer is supervising the surveying of the site in preparation for construction.
Bobby later confirmed that the field was what the numbers referred to and explained what was going on: the field was where a state of the art slaughterhouse was going to be built for humanity. At the slaughterhouse, humans who were dumbed down by the Leviathan food-additive would be slaughtered for food for the Leviathans. By that point construction had begun, which Edgar is later shown to oversee.
Fan theories on the meaning of the numbers
Fans immediately started speculating on the significance of the numbers. Feel free to add your theories here:
- Latitude and longitude shows the position about 2.1 miles east of Mattoon, Wisconsin according to Google in reference to GeoCaching. 
- The latitude 45.4 and longitude -89.5 near to the coordinates for Beaver Lake, in Lincoln County in Wisconsin, which could be one of the spots marked on Roman's map. source
- 454895 is the number of a chimpanzee gene sequence. The Latin name for chimpanzees is Pan troglodytes, and the day the episode aired Jared posted a video of himself smashing his phone called "Trog Break Phone".
- If you replace the numbers with their corresponding letters it becomes DEDHIE which is an anagram for He Died
- It is the air speed velocity of an unladen swallow. Source
- In Google Maps it's the coordinates for Singapore Source.
- Combination to a lock
- 454895 is the Nomen number of Capparis spinosa, the caper bush. The bark and leaves can cause goitrogenic (anti-thyroid) activity, which lead to abnormal weight gain, tiredness, baldness, and cold intolerance. This could be what the Leviathans were using in the Turducken slammers. sources
- On food.com it is the recipe for Hot Italian Sausage Casserole. source.
- Shorthand for the years 1945, 1948 and 1995. Or 454 BCE and 895 BCE
- An alarm system code to get them past security at the Leviathan HQ
- Coordinates to a whiskey fountain.
- The combination to a box hidden somewhere in Bobby’s house that holds a tape, and recorded on that tape is one of Bobby’s bitch rants that verbally slap Sam and Dean’s ass in order all the time, so they’ll have it for when they can no longer just call Bobby up and ask for his advice.
- The combination to a box that is holding a piece of paper that says, "Idjits".
- The number of beers Bobby drinks in a month.
- The coordinates to where Cas is
- A reference to Inception (some fans believe this entire episode was a "dream within a dream" sequence)
- Gematria is a Hebrew system of assigning letters to numbers and things, it's the kind of thing Bobby would know about. 454895 spells out הטחדהד - Daled, He, Daled, Heth, Teth, He- which when translated loosely into English is D, E, D, H, T, E. This could be an anagram of a lot of things, most notably "the ded" and "edeth d," which could be a name.
- Daleth translates as “door”, Heh as “window, Cheth as “field or fence”, Teth as “serpent” (which seems significant). If read right to left from Hebrew it would be, in essence, “window serpent fence door window door”. With the repetition, it could be a very simplistic spell. source.
- It's the hex code # to the color of Bobby's hat. Source
- The numbers in Japanese - a language Bobby is familiar with - are 四五四八九五
- Castiel's ID on a social networking site source.
- The numbers could refer to passages from the New Testament. John 5:4 (At certain times an angel of the Lord would go down into the pool and stir up the water, and whoever stepped in first after the stirring of the water was healed of whatever disease he had) corresponds with 454, and 2 Corinthians 9:5 (Therefore, I thought it necessary to urge these brothers to visit you ahead of me, to make arrangements in advance for this gift you promised, and to have it ready as something given generously and not forced) corresponds with 895.
- As of last Monday, my locker code :D
- The number of Dick Roman's off-shore bank account in the Cayman Islands.
- Bobby's account number with bustyasianbeauties.com. They'll cancel his subscription if the Dynamic Duo don't renew it.
- If they are coordinates in WI, it's not that far away from Plainfield, WI where Ed Gein lived. Maybe they are starting a lampshade company.
- It's the number to God's pre-paid phone.