Hi, Im no expert on SDL,... but just reading that line of code im fairly sure what the code is doing,... but not so sure as to what the result will be.
apply_surface( ( SCREEN_WIDTH - message->w ) / 2, ( SCREEN_HEIGHT - message->h ) / 2, message, screen );
//break this down
( SCREEN_WIDTH - message->w ) / 2
// SCREEN_WIDTH is the with of the screen(drawing canvas) you are using. its taking from that the width of the message its going to show and dividing the result by 2.
( SCREEN_HEIGHT - message->h ) / 2
// this is doing the same but with th height .
My guess is that either this is going to center a message about the origin (hence the divide by 2 to get half the width as half goes on either side)
or it wants to know the border area around the message if it was centered for whatever reason.
But like i say im no SDL expert but that to me is what it is doing .