tagged information from your music collection into a MySQL-database. A lot of configurable options exists in order to fine-tune the operation. Read more
Can I upload and download music ?
Short answer: Yes - if you're allowed to do so...
If configured+enabled and only if you have permissions to do it, you might see an option in the personal settings to upload and/or download music.
When enabling (checking) "Show download option", you'll see a "Download" link next to all tracks, albums, performers, favorite lists, the queue etc.etc.
When enabling (checking) "Show upload option", you'll see an "Upload" link in the menu to the left, that makes it possible to do a primitive upload of one or several tracks.
More information here:
Configuration of download.
Configuration of upload.
Setting permissions for each user in relation to enabling/disabling upload/download.