It's called reverie. The stream of thinking that you naturally have when you're not deliberately trying to focus.
Essentially, one thought leading to another which leads to another and so on, without you having to make any effort.
Most people identify so much with their thinking, they aren't aware of it as a "thing"; it feels like it's something you're doing.
What are you thinking about?
But if you try to just pay attention, say to your breath, going in and out, after a few seconds you'll discover you slipped back into thinking, as described above, back into reverie.
Continued failure, despite your increasing resolve, to stay just listening to your breath reveals this fact: you can't stop "your" thinking mind from recapturing your attention.
So it seems like a monkey, that is, a thing which jumps around, from thought to thought, a separate entity from you, one which will not obey you.
