"String" is a geek word for text. A string of letters. A string expression is about text, like characters (ABC… abc…), digits (0123456789), and special characters (!#*>]{…).

String constants

Remember that numeric constants are things like 33, -1.009, and 32.22. Numeric constants don't change.

There are string constants as well. They have " around them, like "This is a string constant". The stuff inside the " " will never change.

Another example:

line1 = "Who let the dogs out?"

The thing in the quotes is a constant. It will never change.