what is the use of @InvocableVariable annotation in salesforce?

Use the InvocableVariable annotation to identify variables used by invocable methods in custom classes.
The InvocableVariable annotation identifies a class variable used as an input or output parameter for an InvocableMethod method’s invocable action. If you create your own custom class to use as the input or output to an invocable method, you can annotate individual class member variables to make them available to the method.