Usually used in reference to "extreme spread"; referring to the difference in velocity between the fastest and slowest bullets for a given handload. You need a chronograph to measure the velocity of your loads. Guys will also use SD as an abbreviation of Standard Deviation. Another measure of how consistent your bullet velocities are.
With respect to ES and SD, smaller is better. If your loads are more consistent in their velocity, you will see less vertical dispersion in bullet impact as you begin shooting farther and farther. A high ES or SD won't make much difference in your group size at 100 or 200 yards. As you get out past 400 yards it can make a difference.
If you see the words "maximum spread", they're probably referring to the group size on the target, measured as the distance between to 2 farthest apart bullet holes on the target. Most folks use the "center to center" distance, which can be measured by taking the distance from the outside of the bullet holes and subtracting one bullet diameter.
Sorry if I sound patronizing. Just wanted to cover the basics.
Hmmm, I see while I've been typing, liltank has already answered your question . . . . .