www.office.com/setup Blogs: What is a channel?
Channels allow teammates to have focused conversations about specific topics. Every team has its own set of channels (starting with the General channel), and the members of that team get to decide what they want the rest of their channels to focus on. Some channels are devoted to specific projects, some are for areas of interest. Some channels are just for fun. Every channel belonging to a team is completely public to all the team members, so everyone can feel free to join in a channel conversation.
If you spot a channel that looks interesting or is relevant to you, you have the option of following (or, favoriting) it. You’ll always have the General channel for every team favorited and there's no way to remove it from your favorites, either.
To favorite a channel, just click the star icon next to a channel name. That way, it will stay visible in your teams list. You'll also be able to see if there's recent activity in your favorite channel because its name will appear in bold text. If someone @mentions you in one of your favorite channels, you'll see a red number count next to the channel name that tells you the number of @mentions you received.
You won't be able to see the team channels that you don't favorite by default, but if you ever want to take a look at all the other channels belonging to a team, click on the text that appears at the bottom of your current favorites telling you how many more channels belong to that team. That will open up a list of all the channels you haven't favorited. If you see a channel that looks interesting, you've got the option of favoriting it from there.
Everyone on a team that's following a specific channel will be able to see if that channel is active thanks to the bold channel name, but they will only get notifications if they're specifically @mentioned within that channel. If you get @mentioned in a channel you're not following, you'll get a notification in your Activity feed.