Built-in variables are a special category of variables that are pre-created and non-customizable. They replace the variables that used to be generated when creating a new container.
New containers have no user-defined variables at the start, and some built-in variables will be pre-enabled. Users can quickly enable any others that they'll need.
Enabled built-in variables will be made available throughout Tag Manager in such places as the Variable Selection drop-down menu. Once a selection of built-in variables have been enabled, you can use them just like any other variable.