Here's the wiki page.
It's sticky and makes pulling apart the frames difficult, which, of course, is the point. Look at the length of the pull apart - and it's staying stiff even in the heat. It's a pretty great adhesive.
I was told that propolis was greenish black and one of our hives does indeed have greenish black propolis, but the other has screaming orange propolis.
The color has to do with the type of tree they get the sap from.
Bees will use propolis to seal up anything they don't want in the hive. They'll even use it to mummify a dead mouse with. I've seen video of bees sealing off a bunch of Small Hive Beetles with it.
Rumor has it that it's good for you. If you take it, swallow it whole - don't chew it because it'll get on your teeth and won't want to come off. Like trying to take pine sap off your hands. You don't want to go there.
You can even use it for art projects. Lily made this little rose out
of the propolis I was scraping off the frames at the last inspection.
Weird, but cute.