How to check whether an user visited our site since some days

Hi, I know this is not the correct place to ask this type of question but I find this feature only on this site. I haven’t visited the hugo discussion site since some days and when I visited today it shows a message that so much stuff is there. How did they track my presence in their site. I am very excited to know about this .

This is the “meta” channel for Discourse, which is the forum software being run here: