In short, @mentions are an easy way to draw someone's attention to a high-priority conversation or message.
To mention someone, you need to type @theirFleepusername in the message. You can start by typing the @ character — a popup menu will appear with a list of all users in the conversation.
If there are a lot of members, you may want to type the first few letters of their name or username — the popup list will then only show matching results.
How will the mentions look like to the other person?
Mentions are visible in the message flow and in the conversation list. When you notify someone with an @mention in the conversation, they will see an @ sign next to the conversation's name in the conversation list.
Furthermore, if the persion you @mention has push notifications enabled, @mentions will always trigger a notification to them (even if there are previously unread messages in that conversation and even if the conversation has been muted).