If you have to make changes to something, you make a readjustment. You make readjustments all the time — like when you lower the bike seat for your little sister or set your clock forward for daylight saving time.
The noun readjustment refers to the act of recalibrating something to match a standard. If you are a scientist, you may be constantly making readjustments to your instruments in order to obtain accurate data. If you're at home, you may have to make some readjustments to your television recording device. The clock is set wrong again and it keeps cutting off the end of your favorite shows, which is super annoying.