What is a safety number?
Each Signal conversation has a unique safety number. It is a 60-digit number that let's you verify your privacy with a specific contact. To view the number, open the conversation with your contact, tap on conversation settings, and choose verify safety number. You can then compare the numbers or scan a single QR code to make sure it is the same number on both your phone and your contact's phone. If the safety number is identical then you can be sure that you are communicating with the right person.
Why do I see a safety number change alert?
You will see this alert because your contact's number is used on a new install of Signal since the last time you chatted. They probably re-installed Signal or got a new phone.