What does CG_INLINE do?

I was poking around the definitions for things like CGPoint for hints on how to create my own functions but I don't know the purpose of CG_INLINE. What is happening behind the scenes here?

CG_INLINE CGPoint
CGPointMake(CGFloat x, CGFloat y)
{
  CGPoint p; p.x = x; p.y = y; return p;
}

CG_INLINE CGSize
CGSizeMake(CGFloat width, CGFloat height)
{
  CGSize size; size.width = width; size.height = height; return size;
}