From what I understand, which arguably isn't much, rektbot publishes data from exchanges where people borrow money to buy stuff that they can potentially sell in the future for profit for a lot more than if they just used their own money.

But since no one can predict the future, those nice outcomes don't happen very often, and the bad outcomes are more likely, and people end up in a position where they both lose all their money and have to pay back all the money they borrowed. Hence "getting rekt."