Hacker pool, i still don't get it...

Hi guys. Let me tell you something: I don't know about the hacker pool! This may sound silly but can someone tell me what is the hacker pool? Because i am curious to know! I heard it from the Bloons TD Battles/Strategies page and tried google-searching for it but it still didn't work. I will say this one last time: Can someone please tell me what is the hacker pool?