Local variables hold data local to a screen or to an action. Local variables only exist in the scope of the screen or action where they were created. Local variables should be used whenever there are no external dependencies between that action flow, for example, and the rest of the application. They are usually a repository for temporary data.
A screen local variable is instantiated before the preparation and rendering of that screen.
You can set a default value for your local variables through the Default Value property. See more about how to set a default value for the local variables.
Once the local variable is created, it is accessible in the Select Variable window and can be used in the Expression editor. In both editors, the local variables are displayed in the Locals folder.
About Variables | Local Variable Properties