A channel audit is an occasional run through of all of the channels you've joined and deciding whether you still need to be in them. In general, if you haven't contributed to a channel and/or have had it muted for >1 month, you might want to consider leaving.
To leave a channel, select it in the list, then navigate to More options > Leave the team.
If you come across a channel that you own that has gone dormant (meaning that no one has been posting for >1 month), please mark the channel as Archived by renaming it ARCHIVED-NAME so that no one will mistakenly post in it. (See MS Teams Channel Naming Conventions)
To change the name of a channel, select it in the list, then navigate to More options > Edit this channel.