Butt plugs (or anal plugs) are sex toys designed to be inserted in the anus and rectum for sexual pleasure. In some ways, they are similar to a dildo, but they tend to be shorter, and to have a flared end to prevent the device from being lost inside the rectum. (Many dildos lack a flared end, and thus it is ill-advised to use such dildos anally, since they may get stuck, requiring medical extraction.)
People of all genders and sexual orientations may use butt plugs.
Unlike the vagina, which is closed off by the cervix, the rectum leads to the sigmoid colon. Objects which are inserted into the rectum can therefore potentially travel up into the bowel: the flared end on a butt plug exists to prevent this.