Whatever information that had been told to you, enters your mind and "exits", causing you to forget immediately/after sometime. Often used with ears.
Teacher: Tom, what is the solution to this problem?
Tom: Don't know.
Teacher: Are you even paying attention in class for the past 5 minutes?
Tom: Yes but its 1 in 1 out right now.
Teacher: What do you mean?
Tom: Whatever you said, enters through 1 of my ears and exits throughout the other.