If someone is self-absorbed, they turn almost all their attention inward: their feelings, their plans, the things that happen to them. Other people may be in the room, but this person does not really take them in. It is not a kind word. You use it when you think someone is stuck in themselves, not merely shy, and not merely busy. It sits near “selfish,” but it is not the same: selfish often means you take, or you will not share; self-absorbed means you barely notice anyone else is there. It is a judging word, a little formal in shape, but people say it in ordinary talk. Younger speakers use it a lot about phones and social media — someone watching their own life as if it were the only show in the room. You can say it of a moment (“He was self-absorbed all evening”) or of a habit.