Blossom Servers are very basic binary file storage endpoints. The protocol allows you to mirror your files across multiple servers.

Then when you create any event on Nostr, depending on which client you use, the client will include URL to all your mirrored copies of your files. So, let's say someone comes across your 2-year-old Nostr event, and they first Blossom server is long dead, it might still work if the second or third Blossom server is still running.

Great for uploading and storing podcasts, music, etc. They can also transcode uploaded videos to smaller sizes, or photographs.