A ripple is a small wave on the surface of something, such as a ripple that forms a ring around the spot where you threw a pebble into the pond.
Ripple can also be a verb. For example, you might see raindrops ripple on top of a puddle. Ripple was first used in the 1670s to describe something that causes a ruffled surface, sort of like how water seems to momentarily gain texture as it's stirred. Ripple later came to describe a very small wave, but you've probably heard of the "ripple effect" in which tiny waves spread, setting off more and more waves.