Muting a topic prevents any question tagged with that topic from appearing in your feed.
You can mute a topic by visiting the Topic page, clicking the "..." beneath the topic name, then choosing "Mute Topic":
On the web: When a topic is muted the checkbox next to "Mute Topic" will be checked. To Unmute, click the checkbox to remove the check.
On our mobile app: When a topic is Muted, the text in the menu will change to say Unmute. Tapping on Unmute will allow questions tagged with that topic to once again appear in your feed.