In the Details field to the right in a Ticket, you find Labels & Tags. Here you can tag a Ticket by typing the text you want the Ticket to have. Unlike labels, you can type whatever you want to in the Tags field. 

When you tag a Ticket, you tag the e-mail address, you could say – the tag will always be applied to all Tickets from the same sender. So, the next time you receive an e-mail from the same e-mail address, the Ticket automatically will get the same tag. You can tag the Ticket with a phone number or VIP, for example, so that you get some pre information about the sender.

You can also create your own inbox/view below My Views if you want all Tickets with the same tag to be gathered.