Each view has its own coordinate system – with an origin at 0,0 and a width and height. This is described in the bounds rectangle of the view. The frame of the view, however, will have its origin at the point within the bounds rectangle of its superview.
The outermost view of your view hierarchy has it’s origin at 0,0 which corresponds to the top left of the screen in iOS.
If you add a subview at 20,30 to this view, then a point at 0,0 in the subview corresponds to a point at 20,30 in the superview. This conversion is what those methods are doing.
Your example above is pointless (no pun intended) since it converts a point from a view to itself, so nothing will happen. You would more commonly find out where some point of a view was in relation to its superview – to test if a view was moving off the screen, for example:
CGPoint originInSuperview = [superview convertPoint:CGPointZero fromView:subview];
The “receiver” is a standard objective-c term for the object that is receiving the message (methods are also known as messages) so in my example here the receiver is superview.