heads-up — advance warning or notice about something

a construction worker in a hard hat waves his arms urgently toward his coworker who is walking beneath a crane. The first worker points upward at a heavy steel beam swinging dangerously overhead. His colleague immediately stops and looks up, then quickly steps back to safety. The crane operator adjusts the beam's path away from where the worker was walking. Other construction workers nearby turn to look at the close call that was prevented. advance warning being given to prevent an accident from happening