Basically, they're small mods for Stock or DSL that replace the AI (e.g. Emergency or Sentinel) with much more difficult AI (e.g. Neglected Waterbug (Starcore AI v4 Alpha, or SCv4, stock only) or Hope No More 4 (NAR AI v?, DSL only). For pics and general info on most 'famous' bots, check the RA2 Wiki.)
Edit:
Or, in more simple terms, the bots that you fight against are referred to as AI.
AI packs replace these bots with much better bots, such as Neglected Waterbug or Hope No More 4.
The main AI packs;
Stock:
Starcore (versions 1-3) medium
Starcore (version 4 alpha) hard
Inf AI - Very hard
DSL
NAR AI (All versions) Medium
123STW AI - Very Hard