It’s similar to Facebook, where you do @ and then their name to tag a person in specific posts or comments.
Tagging for posts and comments is different than tagging a person. It is basically categorizing them so that Medium puts them in front of readers who follow those categories.
On Medium, stories and comments are the same basic thing. Every comment is also a story, so it can be tagged. As you type a comment, you’ll see “add a tag” at the bottom of the text box and you can categorize it as anything you want.