This feature lets you mention a channel name in the chat itself.
How it works:
Type @ in the chat and your channels in the sidebar will be displayed. You can click on any one of them to mention them in the channel.
In a group channel, you can mention a group member, or any member even if they are not a member of the channel.
This feature helps one easily provide the name of the channel itself without typing the entire name out.
If you want a particular user to join a channel, you can use the @ mention channel feature to directly provide them with the link to the channel.
Refer this video to see more in details - https://youtu.be/ftGOQMK4LQQ