Hello,
A P.R.O. is an organization that collects performance royalties on your behalf (for example, if your music is played on TV).
Up until now, music uploaded on AJ couldn’t be registered with a PRO. This is going to change on October 4th. After that day, you will be able to register your music and collect royalties when broadcasted.
This not an obligation. If you do not wish or can’t join a PRO, there will be no problem for your account. However, you need to state whether your music is PRO registered or not, on the setting page of your Audiojungle account. If you do not select any options on that page, your account will be disabled.