If you think that gathering milk is just easy since it is readily available in the market then you are wrong. Before it can get to the market, the farmer need to feed the cow with lots of nutritious grass and let it drink lots of clean water. It would take a whole lot of labor just to produce a bottle of milk. That is why; every drop of those milk is important for the farmer as they know firsthand the work needs to be done just to produce those milk. And they would not want it to be wasted while it is being delivered to the market. They would invest on the truck needed for the milk to be delivered to the market on time.
In this game, you need to deliver the milk to the market as fast as you can. Don't worry about the time limit as it is nothing serious it is just for additional points and no effect on the actual game. Try to balance your truck as much as possible, so you won't lose any milk along the way. You would notice that the level become more challenging as you move from one level to another.