The snap is when your cursor selects a point such as an intersection, midpoint, centre of a circle, end of line or a vertex. So select a snap point on the moving part and move it to the other snap point. If you want to go to the centre of a square block there is not a snap point. So draw a line from each opposite corner and where they cross will now have an intersection. Use this to snap to and delete the lines when done.
It really easier to do then explain. Keep trying.
Bruce