'Pre-series' is the term used to refer to the period of time in Supernatural before Dean comes to get Sam from Stanford. For example, Sam and Dean as children or teenagers or might describe certain events that have been acknowledged in the canon but have taken place earlier in the timeline (e.g. Sam going away to Stanford University).
The name 'Pre-series' could be considered erroneous, as the opening sequence of the Pilot shows the Winchesters in 1983. Technically, the canon timeline begins here, and there is simply a gap we're not privy to between then and 2006. Alternatively, 4.03 In the Beginning can be seen as pushing the series timeline back to 1973, and 6.18 Frontierland can be seen as pushing the series timeline back to 1861.
Pre-series fic is set in this time period.