If you were in a car which was about to crash you would brace yourself, possibly by pressing your feet forward and pushing against the dashboard in an attempt to stop yourself being thrown through the windscreen.
Bracing yourself isn't always physical. You can brace yourself (prepare yourself ) to receive bad news. So bracing yourself is, pretty much, actively getting ready to withstand something (usually) unpleasant which is about to happen.
Buggles(not a teacher)