It's thinking about what you could / should be doing, and then realizing you've been staring at a wall for half an hour.
It's hearing Step 3 and panicking because you had a question about Step 2, but maybe they'll answer it later on so you don't want to interrupt and- oh we're on Step 7 now.
It's wanting to listen to some music, and spending the next several hours deciding on the perfect playlist.