movie-web was just taken down with all its repos, Yuzu was taken down, then suyu forked it on gitlab and was taken down, countless clones of nintendo games, platform emulators, and a bunch of other things are taken down because they are hosted on the clear web.
If you're a dev and planning to write software for piracy, host it on I2P!
But in general terms, it's anonymization network. Data is sent over multiple hops/nodes and the original source is not included. The nodes are hosted by random people. The downside is that the slowest node in the chain of nodes between the source and the target means that's how slow the entire chain is (at least). So, the more nodes, the better for anonymity, and the faster the nodes are, the better for speed.