What is mention in Twitter?

According to the official Twitter Help Center a mention is "any Twitter update that contains "@username" anywhere in the body of the Tweet".

Users will often see hundreds or thousands of tweets a day in their Twitter feed. Mentioning a user in your tweet means they will be able to see it in their "@ Connect" section on the top of the Twitter page.

You would usually mention a user if you are directing the Twitter update (tweet) to that specific user OR you want them in particular to see this tweet.

If you include more than one @username in your tweet, all of those users will see the Tweet in their "@ Connect" section.