@InvocableVariable in Apex is used when Flow invokes or calls the Apex Class and method.

Apex Class:

global class FlowController {
        
    @InvocableMethod( label='Show Outputs' description='Getting multiple values' )
    global static List < FlowOutput > showOutputs( List < FlowInput > inputs ) {
        
        FlowInput objInput = inputs.get( 0 );
        List < FlowOutput > outputs = new List < FlowOutput >();
        FlowOutput objOutput = new FlowOutput();
        objOutput.str3 = objInput.str1;
        objOutput.str4 = objInput.str2;
        outputs.add ( objOutput );
        return outputs;
        
    }
    
    global class FlowInput {
        
        @InvocableVariable
        global String str1;
        
        @InvocableVariable
        global String str2;
        
    }
    
    global class FlowOutput {
        
        @InvocableVariable
        global String str3;
        
        @InvocableVariable
        global String str4;
        
    }
    
    

}

Flow:

Invocable Variable Configuration:

Output: