A website where you can download paywalled scientific literature. Most scientific literature is paywalled by publishers, and costs a real significant amount to read (like 30-50$ per article if you don't have a subscription).
Scihub basically just pirates it. And has been shut down several times. But as most scientific studies are already laid with public money, scihub isn't that unethical at all.
Lots of scientists will just send you their article if you email them. They don’t get the money when you pay to read it - often they pay to submit. Reviewing journal articles is a privilege and doesn’t get you paid. The prestige of a scientific article is from the number of times people have cited it. The only “harm” done is that the publisher doesn’t get to make 100% profit for doing nothing.
Journal publishing is mostly a way to extract money from universities. Elsevier and its ilk name whatever price they think a research university can afford.