"Checking in" is simply the process of scanning the QR code in your Peet's app when you make a purchase. To check in, select the "Check In" icon from the bottom navigation bar and simply scan your QR code at checkout.
If placing an order via mobile order ahead, your purchase automatically checks you in, and there are no additional steps needed. If you don't have your phone with you, you can also check in with your phone number.