Mail order is shopping for things you need or want without going to a store or shopping center. It usually involves ordering items online, through the mail, over the phone, or by email. In other words, you don't have to go to the store to buy things, you can buy them online or by mail. You pick out the items you want, pay for them, and they will send it to you. Sometimes, you have to wait a few days for it to arrive, but usually it's much faster than going to the store!