Don't really get what packages mean.

There are a lot of answers in the web about it. Check out this discussion on Stack for example:

http://stackoverflow.com/questions/1088509/what-is-the-purpose-of-defining-a-package-in-a-java-file

Also very simple reason, could be that there are not so many words in English Language to distinct different classes when you use many classes in your application:

The more libraries will you use in your App, or frameworks: like Spring or Hibernate (see courses here at Treehouse) the more you have to decide which class you want to use.

Take "Spring Security Workshop" App 'todotoday'. If I type User class, my IDE will print my following User classes and many many others:

• org.springframework.security.core.userdetails.User;
• org.springframework.boot.autoconfigure.security.User;
• org.apache.tomcat.jni.User;

Now you see that a lot of libraries define the same class "User", and the only way to differentiate is using different packages.

About com.teamtreehouse: it is just style following a convention of package names in Java, please read this:

https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html

Good quote from there

Companies use their reversed Internet domain name to begin their package names—for example, com.example.mypackage for a package named mypackage created by a programmer at example.com.