A custom domain name would be something you would purchase from someplace like Godaddy.com 
Using the domain manager in Admin you can park your own custom domain name and use it for your forum instead of having a smfforfree.com address.   For instance instead of http://mirage.smfforfree2.com you could have http://mirage.com  (if that is available)   

There is information in the domain manager about it.   
IMPORTANT:
Do NOT do anything in the domain manager if you have not purchased a domain name.

Please be aware that the forums are being moved to a new server over the weekend.
If you are considering doing this it would be better to wait until the move is completed.

Forum Url: http://Musicinsanity.smfforfree2.com
Well I got my domain from .tk But both of the websites work

It was free



sibyl edit:  blatant advertising deleted