This is the main site, https://www.slsknet.org/news/faq-page I'm linking you to the FAQ. You can get the client there too.

Concept is simple. It's searchable p2p file sharing. You have the client, open/filter two ports on your firewall (soulseek qt has a button to check the ports are open), share a folder with media you want to share with others, then start searching for things you'd like. You can filter searches too. I keep my searches filtered to flac files for example.

As a note, some people are weird about you downloading too much in one go, so good rule of thumb is to DL no more than 1 album at a time from someone. Some people are stupid weird about it though and will block access if you queue more than a single file. Just put them on your block list too and unshare from them and move on. It's not worth it to message people like that.