What does the "mute / (un)mute" link do?

For logged in users, every music link has a "mute" or an "(un)mute"
button, depending on whether you've previously muted the link or not. If
you don't like an artist, you can "mute" it, and it won't show up in your
rankings. Additionally, muted artists won't show up in your recommended
list.

You can still visit the music link by clicking "My Account" in the menu
bar, then the "Muted" tab. There you will see a list of all the artists you
have muted. To (un)mute an artist, click its link in your muted list to
visit the artist's page, then click (un)mute and vou-ala, the link will
show up in your rankings again.