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.
You can read more here: