A lot of things, especially music or TV being played in another room.
My best studying was done late at night after everyone went to bed, or I would go to the large county library and sit at a table on the second floor in the corner that was away from everything. I know I have some ADD, but back in the 80s & 90s that was not an acknowledged thing. Still today I can’t read, pay bills or do anything that requires concentration unless I have quiet.