base stats are numbers, different for each species of pokemon, that are used to calculate the stats of a pokemon. for all stats other than HP, the stat formula is:
(((base stat * 2)+IV+(mathfloor(EV/4)))+5)*Nature
and the formula for HP is:
(((base stat * 2)+IV+(mathfloor(EV/4)))+10+Level)
mathfloor means round down. so mathfloor(43.75) = 43, nature is .9, 1, or 1.1, depending on whether the pokemon's nature boosts or lowers that stat (or leaves it alone), and EVs and IVs are explained in the IV/EV sticky.