How do I read the history of my reputation?
I've just seen -20 in my reputation tab with label please delete; rescinded.
What does it mean? Is it about a question where a user asked to remove it?
It looks like you posted an answer, that had gotten two upvotes, to a question whose author later retitled it to "(please delete; rescinded)". The question was then deleted, and your answer with it, so you have lost the rep from those upvotes.
Since there's no way to search for deleted questions, that's all we can tell from this entry; you'll probably have to wait for a moderator if you need more information.