"Mail-tag consists of an exchange of postal correspondence between (2) people. The first person begins the game by requesting one or more desired things. That person then mails those requests to the other participant - who interprets those requests, writes a list of their own requests for the other participant and sends them back. You can take as much or as little time to compose and send back your reply, ask for a few or as many things in your requests and generally interpret requests in whichever way you see fit."
Basically, you just write some questions or prompts out and enclose them with your letter. It's an easy way to remind yourself that an epistolary exchange should be a two-way conversation. I'm bad at this; I always forget to ask questions. Mail Tagging allows me to give my receipient something to, well, respond to. Thanks for the letters, Rin, and the idea!