You could readjust your TV antenna, your thinking, or the collar of your shirt. Whatever it is, when you readjust something, you make slight changes again and again until you get it right.
When you adjust something, you make a change in order to improve something, such as a driver adjusting the side-view mirror on a car in order to see other cars better. So, when you add the prefix re-, it means "to adjust over and over." In the case of the mirror, you would readjust its angle until you can see perfectly well from your seat in the car.