Freelance is just a way of saying you're self-employed and/or short-term contract commitment. Anyone can declare themselves as a freelancer in any unlicensed profession, but I'm guessing what you want to know is what it means and what to expect being a self-employed writer?
There's not really a need-to-know thing about freelancing. You can pick up a writing gig off the recruitment page or some other website and you'll be freelancing already. People or parties offering writing opportunities may or may not ask for samples or have you work on a prompt before deciding to work with you. It wouldn't hurt to have samples on hand as it can demonstrate the creative breadth of your writing. It also wouldn't hurt to have some experience, because experience always looks good.
If you're fresh, and I mean fresh and green around the ears to writing and want to pursue some experience, I would suggest searching viewforum.php?f=59
There are a plethora of projects, and for most people, offering your services free is the quickest way to accrue experience for yourself and discover the kinks within your own writing. Also, seek small projects. By small, I mean projects that are well-defined in their goals that can be completed within a reasonably short amount of time. That way, there is little risk of the project failing.
I guess, if I had to give advice from my own experience with freelancing, is to be VERY CLEAR your capability. If the party in question asks you for xxxx amount of words in xxxx amount of days, be sure you can complete it or negotiate a more convenient work schedule. Same goes with pricing your own rates, know your worth. Granted, that usually comes once you've gained enough experience that the quality of your time and efforts means something.
TL;DR:
- You can freelance right away if you really want.
- Experience not necessary, but is always good
- Seek many avenues for greater number of opportunities
- Don't be afraid to start on very small projects.
- Negotiate, negotiate, negotiate
- Don't be greedy, but don't sell yourself short.