What is what does by phone number mean on snapchat?

When you add someone on Snapchat using their phone number, it means that you have synced your phone's contacts with your Snapchat account and are able to see who is on Snapchat that also has their phone number associated with their account. This feature allows you to easily connect with people you know in your phonebook without having to manually search for their Snapchat username. Additionally, if someone has added you using your phone number, you will receive a notification and can choose to add them back or ignore the request.