When you're dealing with an indefinite amount of something, you will use the partitive case. This is especially common with
What is a partitive example?
A partitive noun is a noun that is used to describe a part or quantity of something. Partitive nouns are used with another noun to tell you how much of that noun there is. "Boxes" in "three boxes of cereal" is a partitive noun because it describes a specific quantity of cereal.
What is the partitive case in Russian?
The partitive genitive in Russian is used when we talk about having part of something. When we talk about eating bread, we (usually) don't mean we eat the entire loaf of bread. Russian also has to make this distinction, and so puts the object into the genitive. Он съел хлеба.