It's a friendly warning, a way of saying "pay attention" or "be aware of your surroundings."
Here are some examples of how you might use it:
* "Pas jou op vir die karre!" (Watch out for the cars!)
* "Pas jou op, die trap is glad." (Be careful, the stairs are slippery.)
* "Pas jou op vir die hond, hy byt." (Watch out for the dog, he bites.)