Back-to-back things come right after each other. Two back-to-back baseball games are played in a row.
Back-to-back has been used to describe houses that are literally back-to-back with no separation. These days, back-to-back events happen consecutively. A baseball player with back-to-back home runs hit two in a row. A President with back-to-back victories won two elections in a row. There's no separation between back-to-back things, like how if your back was pressed against someone else’s back, nothing could fit in between. Sometimes people use the term back-to-back-to-back for things that happen three times in a row.