A base stat is the frame for a pokemon's strength in a certain respect. There's a bunch of math that goes into it, but the tl;dr of it is that the higher your pokemon's base stats are, the higher the stats themselves will be. It's not based on anything at all; it's just a set number the game programmers assigned to the pokemon, and it will be the same across the board, meaning that every Zoroark you see will have the same base HP, attack, speed, etc. The actual stats will be different though.
Base stats are the reason some pokemon still don't do well with one stat even if you invest EVs in them. Chansey for instance only has base 5 Attack, so the highest its attack can ever legitimately be is 119. Nothing really impressive, right? That's why you don't see Chansey using physical attacks; its base stat for attack is too low for it to be of much use.
