Do you understand the concept of variables? If not I can explain. A string is a text variable, as opposed to an integer (3, 9, 142, or 71 for example), or float (1.3, 9.189, or 7.4 for example).
This is a string: “Welcome to Fluther!”
This isn’t a string: 9
This is a string: “Hello!”
Strings are enclosed in quotes. The quotes aren’t part of the string itself.