"bot" is short word for "robot".
As a "robot" is the most time something that made something repetitive with no human prompt during its work.
For a game like rappelz and in the case of the one we get here, it's a program that play in place of the player.
You set your parameters, you go where you want the program use its functions, you launch it, and you go somewhere else do something else..
When you come back, the bot have killed some monsters. If you are lucky, it should even get some great drop for you. If it work well and a long time, it should even give great XP to your char.
But don't forget something really important.
If GM learn you use it, you will get a definitive ban.
For all existing game, all bot are out of convention. It is like cheating.