It’s a program that uses an SDR to pick up the signals broadcasted by planes (ADS-B) containing their flight information. Then the data gets uploaded to an aggregator (FR24, Flight Aware, ADS-B Exchange) that gives a global view of all planes in the sky.

You can use the aggregators for free without uploading, but you get some perks for being a contributor. I just do it because it’s cool and I use the platforms for getting info on flights I’m taking (you can tell if your flight is gonna be delayed if the plane is delayed elsewhere for example).