For new screenwriters, there are only three ways of getting your script turned into a movie:
(1) make the movie yourself,
(2) sell the screenplay, and
(3) option it to a producer/director.
An option is the most common way for you to make your money as a screenwriter. An option basically is when the writer is paid money by a production company to temporarily have the right to revise it and use it to raise monies to make it into a movie. An option is like a lease and is a temporary transfer of rights. It also protects the company by giving them assurance that you won't try to sell it to someone else while they're busy trying to raise funds. Typically, the option is placed against later purchase. If the company later buys your script, you received the purchase price minus the option. A typical option is 18 months or a year and half. Most option clauses allow for one extension. The second option does not come out of the purchase price normally. If the option is not exercised by then, all rights revert back to the writer. Often an option price is $5000 for a feature. The purchase price can vary. Unless you have a "golden script", very few production companies will outright purchase a script from an unknown writer. Often a stipulation of an option is the re-write agreement.
In the past, Amazon's policy was very unfavorable. Presently, you can post your screenplay publicly or privately. If posted privately, they will review it for 45 days during which time they will consider it for optioning. At the end of 45 days they will either option the script for $10K, buy it outright for $200K or pass. If they pass, you can remove your script and market it elsewhere. If they option it, they pay you the $10K and can have others revise the script. During that 1 1/2 year period, they will create mock-up versions which they will try to market to other producers and their partner, Warner Brothers Studios. If they choose to buy the script, you will receive $200 K minus the $10 K option fee.
For the screenwriter, even if a production company can not make a film from your script in the option period, you get to keep the option money. This is a positive thing. I have known screenwriters who've never sold their script but who have made money simply from the options. I don't believe Amazon forces submissions to be optioned. However, as the goal is to get your script produced, Amazon Studios is a new approach in that if optioned, they will have others make trial versions of your script and also have others work with you to modify your script. And as author, you are also eligible for additional payments if it is a box office success.
They review your script for 45 days. After that period, you are free to do what you want with your script. If they option it, you will receive $10 K. That's a good thing. You could only be receiving $5 K and still have to give up control for a 1 1/2 years. But for most writers, something is better than nothing. And most writers have lots of scripts to market in the interim. If they love it, they may buy it, but I wouldn't count on that.
There is a lot of competition, so don't plan on the $10 K option quite yet. One way they use to select likely scripts is their "Premise Wars" which is basically a crowd-based rating of loglines. It is a new approach that will probably crop up in Google and other larger service sites. Also they want feature films, not shorts.
DeGrande's comment "... the freedom to do what I want." doesn't really apply in the same way. The freedom to do what? As a screenwriter, you make it into a movie yourself or you send it off for others to evaluate. You might end up waiting 3 months for a reply (usually a formal rejection if you don't have an agent). Every writer learns to be patient. During any submission process you are expected to limit your submissions. Any company who options your script will immediately acquire exclusive rights. Unless you have an agent or you are very well known, don't expect a bidding war to buy your script.
One caveat, every time you revise your script (private or public), you reset the 45 day evaluation period. When you post, be sure you are posting the version you want considered. In fairness to Amazon, some script modifications are substantial and so the original assessment can no longer be used. So even if you think it's minor fixes for spelling, that is a new version and re-starts the 45 day evaluation. And if you post anywhere, be sure to register it first.
I am not providing an endorsement of Amazon Studios, but I find their newest terms more reasonable for the screenwriter. In the past, the terms were far more disadvantageous. It is one avenue for screenwriters to consider.