Focus is whatever’s on top of your attention pyramid. There are certain things that can shove their way up there, which you’ll know from when a light comes on at the edge of a cinema, and distracts you, or when you’re driving and the person in the passenger seat is moving around in the corner of your eye.
These things are attention grabbing.
In general, things that could be dangerous animals coming to eat to, or people coming to attack you—those move up the pyramid fast. Fast motion, especially objects getting bigger, and fast contrast change. Those grab your attention from wherever it’s at to whatever is changing.
So what?