to tell someone that something is going to happen: I just wanted to give you all a heads up that we will be talking about the first two chapters of the book tomorrow.

Can give a heads up meaning?

: a message that alerts or prepares : warning gave him a heads-up that an investigation was pending. heads up. interjection. Definition of heads up (Entry 3 of 3) —used as a warning to look out for danger especially overhead or to clear a passageway.

Is it OK to say heads up?

As you said, the term “heads up” is informal. However, it is so common in American English that we use it in almost every situation. “Heads up” can be used as a noun. It sends a message that says something is going to happen.