A JAR (Java Archive) is a file that contains a collection of classes. As an example, let's say sample.jar file has the following. This means that sample.jar has two classes. One class is com/sample/main/People (or com.sample.main.People) and the other class is com/sample/main/Places (or com.sample.main.Places).
Apps can be configured to use one or more of the classes in the JAR. For an app to be able to use a class in the JAR, the app needs to be able to access the JAR. There are 3 ways an app can access the JAR.
Each of these options are described in more depth in this article.