F # function parameters

I have 2 lines of code here:

let stuff _ _ = MessageBox.Show("This is doing stuff") |> ignore
let stuffHandler = new EventHandler(stuff)

      

        
        
        
      

    

I just started with F # and would like to know why the code doesn't compile if I remove the underscores from the substance variable? As far as I know, underline means to ignore something that should be ignored here?