You missed a lot of the information given in the answer. Here once again a snipped of what Sun defined:
The prefix of a unique package name is always written in all-lowercase ASCII letters and should be one of the top-level domain names, currently com, edu, gov, mil, net, org, or one of the English two-letter codes identifying countries as specified in ISO Standard 3166, 1981.
Today we have even more top-level domains. The important part is to choose the domain you own in reverse order. I suggest you should read the answer once again, slowly. The goal is to avoid naming conflicts by choosing unique namespaces. And because domain names are already owned by a single company / person, they are good candidates to choose.