The gradient of a surface is its slope. If you're a daredevil and you're looking for a road to fly down on your skateboard, you'll want to find one with a fairly steep gradient.
In mathematics, the gradient tells you how steep a line in a graph is. In physics, when you say gradient, you're talking about how quickly something changes from one point to another. The word comes ultimately from the Latin gradus "step," and a gradient gives you a measure of the "steps" by which something changes.