Answer:
Runoff is one of the ways land water finds its way back to the ocean, so it can join the water cycle again.
Usually, all the water that falls to the land gets soaked in the soil. However, during long rainy periods, or in the spring, when snow and ice glaciers start melting, soil gets oversaturated with water, so this excess water start flowing on the earth's surface. Gravity directs this water from higher to lower areas, where it will flow into some valley river. This river will take the runoff water to another river, then to the sea or the ocean.
Now this water can evaporate and start the water cycle once more.