Directly under that in the Discoveries section you'll find the following:
"Roll a d6 and divide by two, then round up, to determine the number of hexes to color in for the above discoveries."
So if you roll your d20 and get a 2-12 on a discovery you will also roll a d6 to find out how many hexes you'll be coloring in.
For example: You roll a 2 - which is a small water source and all the hexes for this will be water. Now you roll a d6 and find out how many hexes. You then roll a 3. Divided by 2 that would be 1.5 so you round that up to 2 and color in 2 water hexes.
We will be updating that section to:
Please feel free to ask if you still need more clarification.
~Angel