Example structure of an invocable apex class

InvocableApexTemplate.cls

public with sharing class InvocableApexTemplate {

    @InvocableMethod(
        label = 'Name as displayed in Process Builder'
        description = 'Tooltip as displayed in Process Builder'
    )
    public static List<Response> execute( List<Request> requests ) {
      
        List<Response> responses = new List<Response>();
      
        for ( Request req : requests ) {
            
            // do something with req.userID
            
            Response res = new Response();
            res.outputValue = 'something you calculate';
          
            responses.add( res );
                
        }
        
        return responses;
    }
    
    // ---------------------------------------

    public class Request {

        /*
         * Each @InvocableVariable defined in this class
         * is an input variable you can populate when calling
         * this from Process Builder or Flow.
         */

        @InvocableVariable(
            label = 'Label of my input variable'
            description = 'Tooltip for this input variable'
            required = true
        )
        public ID userId;

    }
        
    public class Response {

        /*
         * Each @InvocableVariable defined in this class
         * is an output variable whose value you populate
         * in your @InvocableMethod to return back to
         * Process Builder or Flow, although Flow may be
         * the only one that can do something with the response.
         * Process Builder takes more of a "fire and forget" style.
         */

        @InvocableVariable(
            label = 'Label of my output variable'
            description = 'Tooltip for this output variable'
        )
        public String outputValue;

    }
            
}