It just means they are logged on to the site. If you click on the little green icon where it says "online," a chat box will open and you can chat with them. They may or may not chat back. They may be busy or chatting with someone else or logged on but not physically at the computer. Some providers use that feature to set up dates. The online status will show up on your favorites, when you are looking at reviews, and also if you click on "chat with provider" at the top of the screen in the reviews submenu it will show you all the ladies who are online.

I always choose to appear offline because I don't like to chat.  But if I wanted to appear online, the option is in my account settings.