The two numbers represent the “pour point” flow characteristics for oil temperatures of 0 and 100 degrees centigrade.
The “5” in the 5W30 means the oil will have the same “pour point” flow characteristics as a straight 5 weight oil at 0 degrees C.
The “30” in the 5W30 means the oil will have the same “pour point” flow characteristics as a straight 30 weight oil at 100 degrees C.