WGTNiv answered that question here: He wrote:
nivlac:The numbers on the green just represent the depth (in yards) of each section of the green. In real life, knowing this information helps you decide if you need to hit a 7I to the front of the green or a 6I to the back.
On the better courses, each one of those "thirds" will have a number 1, 2 or 3. They vary from hole to hole (ie #1 from top to bottom is 1 - 2 - 3, #2 is 2 - 3 - 1, #3 is 3 - 1 - 2, etc through 18) Then when you check in at the clubhouse you will see that for today the pins are in the #2 postion... (meaning in our example, the pin is in the middle third for #1, the top third for #2, the bottom third for #3, etc etc).
Like Niv stated, this way you can know if you need to approach high, low or play for the center.
Hope that's not too confusing.
Peace.