What does "tl;dr" mean?

I was at school today and we were doing this project in my Global Studies cl. We had to read up on a famous inventor from history and then write a five paragraph essay. We were each igned someone different to research. Now my cl consists of about thirty or so kids, so it's a large rather cl. Because of that, we get partners to work with. I got to work with this kid who was new this year because he came from a foreign country. My teacher handed out the reading to us and I was starting to read it when he snatched it from me. He then took a glance at it and said something. This kid is sometimes difficult to understand, but I asked him to repeat himself and he said the letters "t" and "l", then said "colon", "d" and "r". I gave this some thought and couldn't figure out what he meant. I don't think he was talking in another language because he said something about it being an abbreviation for something. I now feel kind of stupid and uncomfortable. So I thought I would ask Yahoo Answers. Please enlighten me and tell me what he meant by that. Also, if it means something bad, I am terribly sorry and I can understand if you would penalize me. But I don't mean this in a bad way, I just want to know what it means in case someone says that to me again. Thanks people. ^__^