A Custom URL in your steam profile is what comes after /id/ in the link.
For example: steamcommunity.com/id/example
You can set this in your profiles settings
Selly link:
Selly - Steam 3 Digit Custom URL
There are only about 12000 custom url's with 3 digits that are unused.
I have them in a list and i'm selling each one for $1.
You can buy them from me on selly and you will automatically receive the 3 digits in your e-mail.
You can change your Custom URL here: Steam Community