can you say what are mentions?

When one website cites another website, that is a brand mention. The SEO community holds the belief that Google will treat a website's mention of another website's domain name or URL as if it were a link. Brand Mentions are another name for an implicit connection.

Using a @mention, you can alert colleagues to your post. Anywhere in the post's body that the @username appears is considered a @mention. To share your news with individual coworkers, use the @mentions feature.