The @ sign is used to call out usernames in Tweets: "Hello @twitter!" People will use your @username to mention you in Tweets, send you a message or link to your profile.

See at

At (referring to price)

The @ sign is used to call out usernames in tweets, like this: Hello @Twitter! When a username is preceded by the @ sign, it becomes a link to a Twitter profile. 

 This sign is used to call out user names in Tweets. E.g. "Hello @Twitter!" When a Twitter handle is preceded by the @ sign, it becomes a link to the Twitter profile.  

The ‘at tag’ is also known as the Anchor name, Twitter username and Twitter handle. This is the name a user chooses for themselves. The ‘at tag’ is used to link that specific account to a tweet. [..]