Plus addressing is a standard feature of email. It allows you to add arbitrary text after the user name. This can be useful in a variety of ways like sorting or identifying the source of the email. I use it sometimes when I sign up for a random service. It’ll let me know they shared my email with a third party if I get emails from other sources with the same +text. Only works for that if they are too lazy to strip the +text, though.

The idea being you can use it like an alias, so if you give youtube the address of "myemail+youtube@gmail.com" then when you get unsolicited spam emails addressed to "myemail+youtube@gmail.com" you know which service sold your information. Probably also makes it easier to sort the inbox based on where it is addressed