What happens when support has ended for a specific version of Ubuntu?
So old versions of Ubuntu are unsupported. What does that mean?
It means that you won't get security updates and selected high-impact bug fixes any longer.
You can continue using your Ubuntu, without security updated and bug fixes, as @htorque said.
I would to add that you can also install new software available in the frozen repositories, simply substituting the server you are using in sources.list with the following
http://old-releases.ubuntu.com/
Needless to say, you are strongly advised to upgrade to a newer release of Ubuntu.