Measure words are used whenever you talk about quantities in Chinese. You can’t attach numbers directly to nouns - you have to put a measure word in between.

There are different measure words for different categories of things. For example, the measure word for books is 本 (běn), whilst the measure word for flat things is 张 (zhāng).

Luckily, there is also a general purpose measure word: 个 (ge). This can be used for pretty much everything, but it’s better to use more specific measure words when you can.