Along with type Number, this is probably the most commonly used type of all.
Data of type String contains what might be thought of as a label in the non-technical world.
You could use a variable of type String to store your name or your telephone number.
Your telephone number
If you store a telephone number as type String, you cannot perform arithmetic using that data.
This is true even if it consists solely of the digits from 0 through 9 inclusive.
You can only perform arithmetic using data of type Number.