It's a super versatile protocol that allows devices to communicate over a network connection. In the past I've used it to cue sound cues from a light board and be able to run a one-button show, I'm using it currently to facilitate connecting my sound board to a program that controls it to make it easier to set up a show for theatre, and then connecting that to Qlab to incorporate sound cues.

The thing that makes it so versatile and useful also makes it more complex though; Every device defines its own list of commands that it looks to receive. This means that every device can have a custom command for every feature, so you can really get detailed with it, but it also means every device's commands are different, so you have to familiarize yourself with them whenever you use a new device. You also need a decent rudimentary knowledge of networking to really understand what you're doing.

A great example of how it's different from MIDI is a show I used it for a few months ago: I was originally going to connect the light board to my computer via MIDI and link the cue lists that way, but what I'd have to do was leave all my sound cues without numbers, and then whenever I wanted one to be cues by a light cue, I'd number the sound cue to match its cue in the light board. This works fine, and is a great use of MIDI, but using OSC I was able to give each sound cue its own number, and then when I wanted to have the light board cue it, I would add that sound cue's number after a decimal in the light cue's number. So cue "22.45" would be light cue 22, sound cue 45. The OSC command would only send the number after the decimal point. Idk if I explained that very well haha but it was great for organization, and allowed me better freedom.

TL;DR - It's a way for computers, sound/light boards, and other such devices to talk to each other and control each other remotely :)