BP when placed after a number (as in 2500 BP) means "years Before the Present". Archaeologists generally use this to refer to dates that were obtained through the radiocarbon method, although not exclusively.
The advantage to using BP is it avoids the whole philosophical debate about whether in this multicultural world of ours it is better to use A.D. and B.C., which are explicit references to Christianity, or to use C.E. (Common Era) and B.C.E. (Before the Common Era), which still, after all, use an approximation of Jesus Christ's birthyear as a starting point.
The only trouble with using BP is--the Present year, of course, changes every twelve months. Since the BP designation was at least originally associated with radiocarbon dating, archaeologists chose the year 1950 as 'the present'. Radiocarbon dating was invented in the late 1940s, and, also, atmospheric nuclear testing was begun in the 1940s, which makes, at least the theory goes, radiocarbon dates after 1950 virtually useless, anyway.
Other Common Calendar Designations
Taylor, Timothy 2008 Pehistory vs. Archaeology: Terms of Engagement. Journal of World Prehistory 2:11-18.