"Heads up" is a phrase used to warn someone of danger or a situation that must be dealt with immediately. In the most literal sense, it is to warn someone that something is hurtling toward them at great speed and they should look up, so as to avoid being hit.

It can also be used in less severe situations where you simply want to give someone a piece of information well ahead of time. For example, "OK, Joe. I just thought I'd give you a heads up on the Smith account."