You are a bucket and you want to water flowers. You do not need a reason, you just do it.
You can fill up the water meter on the left by pressing S in front of a tap. You can water a plant by standing in front of the topmost part and pressing S. A flower is fully watered when it has a flower at the top.
You move left and right with the A and D keys and jump with W. When you are carrying water, you cannot jump as high, because you are heavier. The aim is to water all 10 plants.