Hi,
I've heard that .ga and .tk domains (I don't know if any other) are free. I've also heard that if you don't manage them, or enter in them for a certain time, they "park" them (between quotes because the revenue is not yours) and fill them with advertisements. Is this true?
It is, unfortunately. One drawback of Freedom.
Otherwise they provide a great service, and they give you free tk, ga and cf domains.
So what must you do to retain your domain?
If your site works, due to a static IP, then you have nothing to worry about. If you don't, you must ensure that the dns records are up to date.