You can mention someone you follow or someone who follows you in the caption of your post, or in the comment thread of a post.
To mention someone, type @ followed by their name. Once you start typing, a list of people should start to pop up. Tap on the person you want to mention.
Once selected, you'll see their name entered in your comment in color. When you're done typing your comment, press Send.
The person you mentioned will get a notification letting them know.