For context:  
On 4chan, there’s a board called /qa/ - Questions & Answers. That board is very badly moderated. As >we all know, due to GR15, you are not allowed to post ponies anywhere on 4chan except for /mlp/ and /trash/. But, because moderation on /qa/ is almost non-existent, people have started posting ponies there, which has angered an individual who is known as Lee Goldson. Whenever he sees a pony post, he replies to it with death threats and other mean sayings, sometimes including a link to the image on Derpibooru. But anons want to have fun, they wanted to know how far they can go with baiting Lee, so they started cropping images found on this booru and called them “leebait”/“cheese”. Due to crops not having any ponies in them, they could bypass GR15. The most popular leebait are crops of a foot-fetishistic comic about Don Turtelli from TMNT tickling the feet of Equestria Girls versions of MLP characters. One of his quotes in that comic is “It smell like cheese!”, hence why leebait is sometimes called “cheese”. However, it is not known whenever that “Lee” is a single person or not, or someone else pretending, or a bot replying and finding links to original versions of the crops. Some leebaits on this booru have replies from /qa/ anons that do decide to check the links out.