What IDE and SDL are you using?
Myself, when I don't know some SDL function, I either press on this function name I just entered with right mouse button and press on "Find definition of $funct" - this is provided to me by my IDE(Code::Blocks), but I believe Visual Studio should have something similar.
There is a brief explanation in there, where function is defined, and if you read it, it is generally sufficient. If not - look online.
Back to the question. YOu have function explained here:
Answer to your question is in the very first sentence after function declaration.
&screen->clip_rect is accessing your screen's clip_rect data(it a rectangle containing x,y,w and h of image).
Basically, in lazyfoo's tutorials, screen is of SDL_Surface* type, and you use & and -> to get the type you want:
SDL_Surface* screen; // It's how it's defined in your code, I assume.
auto first = screen->clip_rect; // This will be of type SDL_Rect
auto first = &screen->clip_rect; // This will be of type SDL_Rect* . The type that is needed as parameter in your function ;))