It implies that something challenging or potentially negative is about to happen and you need to mentally and/or physically ready yourself for it. It's like putting on a metaphorical "brace" to withstand the impact.
Here are some examples of how it might be used:
* "Brace yourself, the news is not good." (Implies bad news is coming)
* "Brace yourself for a bumpy ride." (Implies a difficult or uncomfortable journey)
* "Brace yourself for the final exam." (Implies a challenging test is coming)
Essentially, "brace yourself" is a warning to be prepared for something that might be tough.