When someone is giving you a "heads up" he is, actually, giving you a warning (in advance) , he is telling you something he thinks you should know about any kind of situation ( an important situation or not) , It's like a notification, some useful information to keep you in an observant mode and alert , like this : O_O ^^

Saying "heads up!" is like saying "look out!" "look around!" or "look what's happening around you"... things one cannot do when the heads are down. Adding "just" means "no big deal, no biggie, not an emergency". So "just a heads up" or even "a friendly heads up" means you're keeping someone informed of what's happening, even if perhaps they already know. A very amiable phrase.

'a heads up' is an alert or advisement about something, usually with the nuance of something in the near future or pertinent to the recipient's plan. E.g., "You're heading to the park? Just a heads up, it's going to rain sometime today." "You're leaving for work now? Just a heads up, there's heavy construction on the highway."

it's a way of saying 'for your information', or 'just to let you know'; bringing something to your attention.