Grain milk is like regular cow's milk, but it's made from grains instead of cows. Imagine if you wanted milk, but you didn't have any cows around you to get milk from. So you could still make milk by using grains like rice, oats or barley.
It's a little like making soup. You might use chicken or vegetables to make soup, but you can also use grains like rice to make rice soup. To make grain milk, you soak the grains in water for a while - this makes them soft and easier to blend. Then you blend them up really well until they turn into a smooth liquid.
Just like cow's milk, grain milk can be used in lots of ways. You can pour it on cereal or use it in baking, like making cookies or cakes. You can also drink it straight out of a cup. Some people like grain milk because it doesn't have any lactose, the sugar found in cow's milk that some people have trouble digesting.
Grain milk is also a good option if you don't want to eat or drink things that come from animals, because it's completely plant-based.