What this centering line of code means

    //Create a path that is a rounded rectangle
    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:newRect cornerRadius:5.0];
    //Make all subsequent drawing clip to this rounded rectangle
    //I don't understand this code
    [path addClip];
    //Center the image in the thumbnail rectangle
    //How does this block of code helps to center the image in the thumbnail rectangle and where's the thumnnail rectangle
    CGRect projectRect;
    projectRect.size.width = ratio *origImageSize.width;
    projectRect.size.height = ratio *origImageSize.height;
    //Aren't we suppose to make the origin (0,0)?
    projectRect.origin.x = (newRect.size.width - projectRect.size.width)/2.0;
    projectRect.origin.y = (newRect.size.height - projectRect.size.height)/2.0;
    //Draw the image on it
    //Draw image on using projectRect as the frame but where's the super frame, how will the image turned out to be?
    [thumbnail drawInRect:projectRect];

I listed out all my question using comments in the code. Please explain thanks.