When a company overbooks, it sells more tickets or rooms than it really has. Airlines do this a lot: they know some people will miss the plane, so they sell extra seats on purpose. Though actually, if too many people show up, some have to wait for a later flight, and that is when you hear “the flight is overbooked.” Theatres and hotels can do the same with seats and rooms. It is a business word, a little formal, but you will meet it in travel news and at the airport. The “book” here is not a paper book; it is the verb to book, meaning to reserve.