Hi, that number refers to elevation in feet. Typically you can adjust by about 1 yard of distance per 3 ft of elevation. With experience you will find that approaches to the hole are different on different courses and will need to adjust accordingly.
Note that sometimes this number can be given in inches (typical for elevation around the greens). So watch the units... they can change from time to time. -Keith
PS there is also a small green triangle that tells you whether the elevation is up or down, so add or subtract distance depending on whether the green triangle is pointing up or down.