A place where media presented on nostr protocol are stored, compressed to be opened by any client. When you upload a video or image via nistr app you can choose a blossom server that stores it for you. It cannbe one that you run by yourself forever, this way you own your data. You don't want to store everything on the relays for many reasons...

Very interesting. Thank you. Do you have a good link for beginners to run your own relay?