What is the number within the red oval

What does the number within a red oval above the Mylio application icon on my iphone represent?

That is the number of “Unprotected Files” in other words, the number of photos that have not been saved to one or more of your Vaults. In case you don’t know about Vaults, a Vault is any device in your account that has been set up to save all your full resolution/full sizes photos - like a hard drive or a computer with a lot of storage in it.