An Approval Notice, also as known as an I-797, Notice of Action, is the letter that USCIS issues when an application or petition was approved at the USCIS phase. It normally will be the second notice of action letter from USCIS after the case was submitted, which is why it's also knowns as NOA2. The letter contains important information including a priority date and instructions for next step of the process. The letter itself also can serve as proof of certain immigration benefits and sometimes will be required at the interview. You should keep the original document in a safe place.