Let's say you are making an app the sends email and your name is Bob. You could put "com.bob.email". The package name is a way to identify your app from others, so you should make it specific to you. So for example, I make apps under the name BBA, so I would put "com.bba.email".
Package names also don't need to have three parts to them. As long as they start with "com." they should be fine. You could have seven parts, or only two.
Hope this helps explain it to you.