These are sites that allow individuals to post thoughts, opinions, articles, videos, etc., and where other people comment on things that were posted. They often have groups or threads arranged around specific topics. While you can find good information on them, they’re often criticized for allowing or encouraging misinformation, hate speech, and essentially the worst parts of the internet, and they may set off red flags with your professors. Unless you have a reason to be looking for information here, you probably shouldn’t be.
These are about as far away as you can get from academic or scholarly sources. You should assume you can’t trust anything here without independently verifying the source and/or information. It cannot be stressed enough that you need to be cautious, and that the burden of proof will be on you.