a bot is an illegal (usually) plugin for an online game that allows people to cheat.
It will play for them, allowing them to for example do repetitive tasks for days or weeks on end that build up a character or the character's wealth without the player doing anything.
Or it enhances the player's reaction time or perception of the game environment so he can spot prey (iow, other characters to kill and steal from).
Or (the third major kind) it allows the player to play several characters at the same time, so he can control an entire army all at once and again use that to attack other players and steal from them.

The only legitimate bots are run by game companies themselves during testing of games and to combat player controlled bots.