Mentioning a person
Type @ in the composer, start typing a name, and pick the person from the popup. The mention becomes a chip in the message; when you send, that person gets a notification — even if they've muted the channel.
What an @-mention does
A red pill on the channel in their sidebar.
A browser/push notification if their tab is in the background.
An entry in their Mentions feed (sidebar) until they read the message.
The Mentions feed
Click Mentions & reactions in the sidebar to see every recent @-mention across every channel, in one timeline. Helpful when you've been off-shift and want to catch up without scrubbing each channel individually.