Advertising mail is when companies and businesses send letters or brochures to people's mailboxes to try and get them to buy things or use their services.
Imagine if you received a letter in the mail that had colorful pictures and fun phrases on it, inviting you to come check out a new toy store in your town. That's an example of advertising mail.
Sometimes, companies will even send little gifts or samples of their products along with their letters to try and convince you to try their things. For example, a cereal company might send you a little box of their new flavor of cereal to try out.
Advertising mail can be a helpful way to learn about new products and services that you might be interested in. But it can also be annoying if you get too much of it or if the things being advertised aren't really relevant to you.