OGL = Open Gaming License
In OGL/Open: material in OGL status is material others may use royalty free simply by listing original source credit. They can use it as is, modify it, or even make subtle tweaks in just punctuation... It's Open Source material, so it's fair to use with just an attribution of source.
Out OGL/Non-OGL/Closed: Material which is not released for others use. It is material that the author has chosen to keep his copyright rights over.
SRD: System Reference Document. A document with only the open source material and the mandatory license to use the open source materials under the Open Gaming License released by WOTC. It can be just the changes, or all the open content material of the referenced system.
Currently known (by me) ones out there: D20 3.0, D20 3.5, D20 Modern, D20 Future, D20 Anime, D20 Superheroes, Runquest, Runequest Companion, Runequest Monsters, OSRIC. I know there are more, but I don't know the specifics.
STL: System Trademark License. The separate license to use the system trademark(s). D20 has three separate STL's available. THe OGL in a d20 trademark, which simply requires you have complied with the OGL, and are not producing smut, and still use the 1d20 die roll... The D20 STL, which says you can't include certain elements of the SRD, even though they are open content, and must say "Require the use of a core rulebook by Wizards of the Coast." Since these two licenses are "Automatic if complied to" they are pretty fair game for a variety of uses.
And, of course, the D&D STL, which actually is a series of separate licenses, each with custom terms, and various restrictions, all of which are in non-renewal by WOTC's choice.
The Ramifications?
Anyone who chooses can make a product that is compatible with any OGL SRD.
So... Given the Mongoose RQ SRD, I can rewrite my own version of RuneQuest, and rename it "WilPlay100", and sell it. I could do so with as little as repaginating, and filling in the missing elements. Or, I could take as little of it as I want, slap it together with the D20 SRD, and probably make a Palladium Work Alike, and establish, by use of open source "boilerplate" that I'm not infringing on Mr. Simbieda's copyrights.
One extreme of the OGL is OSRIC. It takes the D20 SRD, and modifies it to the point that it's 99% (estimate) compatible with AD&D, and about 20% compatible with d20. Since it uses NO AD&D text, save what filtered through the SRD's, it's a fairly safe use (abuse, even) of the OGL. Since there was demand for more AD&D 1E materials, and for replacement rulebooks, the OSRIC SRD provides almost all the classes and rules needed to play something Almost Exactly Like AD&D 1E. Many monsters names changed, a few are missing, and a couple classes are missing, but all in all, if you need a new PH, download, print and home-bind up a copy of OSRIC, and you're good to go. Since the wordings vary, it's not quite the same, but it is sure darned close.
Castles and Crusades takes the D20 SRD and does a very different approach... C&C plays differently from AD&D, stock D20 (aka D&D 3.0/3.5), and many others, but being as simple as it is, allows using AD&D, D20, and other OGL20 games' adventures and bestiaries with little to no changes.
T20 Traveller morphs parts of CT and parts of D20 together to make a very playable game, with elements familiar to D20 players, but a feel that is VERY traveller. Except for character stats, it's very much Traveller.
True20 takes the D20 SRD, and thins in down quite a bit, and streamlines it for a "lighter rules" approach.
Basic Fantasy provides yet another adaptation of D20, this time to a level of simplicity equivalent to the old Basic/Expert D&D boxes. It isn't a "compatible" ruleset, though, just a very comparable one. Essentially D20 light.
Mongoose took D20 and morphed it into several very different games, all of which are still D20 games, but each of which uses rules differences to highlight setting material elements for very different feels: Judge Dredd, Conan, Bab5, Starship Troopers.
For Traveller:
The SRD and OGL status will mean you can rework it to your own custom setting, with custom tables and all, and rename it and release it. Depending upon the STL terms, you might even be able to label it "Compatible With Traveller"
Don't like the T&C rules? Write a supplement with your own.
Don't like the specific details of table XYZ, write a patched version, and sell it for $1 on DTRPG... and include those chunks needed to make sense of it in rework, too.
Why OGL?
To basically encourage a shared build up of material, from which one can cull the best versions for one's own works without having to negotiate to avoid litigation. By publishing using OGL material, one agrees to identify and relase one's own OGL material in the same product or in a freely available separate SRD. Further, one's already released it to the public domain, so any one can use it.
It also helps to encourage 3rd party developers to use one's system, since they don't have to pay to make compatible materials.