An airborne collision avoidance system, or ACAS, is like a special way for airplanes to talk to each other so they don't crash into each other in the sky.
It's kind of like when you and your friend are riding bikes and you both say "I'm going left!" or "I'm going right!" so you don't bump into each other. Only with airplanes, they use computers and radios to talk to each other instead of just yelling.
The ACAS computer is always looking out for other planes in the sky and if it sees a plane that might get too close, it will send a message to the pilots telling them to turn left or right to avoid it. The pilots have to follow the direction from the ACAS so they don't crash.
Sometimes, if two airplanes are getting too close together, the ACAS will tell both pilots to turn so they avoid each other. It's like a game of follow the leader, but with airplanes!
Overall, the ACAS is really important to keep airplanes safe in the sky and make sure that everybody can have a safe and smooth flight.