Pasteurization
Pasteurization is heating a liquid just enough to kill harmful germs without cooking it.
Think of it this way
A pot of milk sits on a stove. You heat it until it is just warm enough to kill the germs floating inside. The milk never boils, so it still tastes fresh. The germs are gone, but the liquid stays the same.
Explaining pasteurization by grade level
Milk from a cow can have tiny germs in it. Heat kills those germs and makes the milk safe to drink. The milk does not cook or change its taste much. Most milk you buy at the store has been heated this way.