"Global rewards" are rewards that every player in the galaxy receives, or can access. They are usually stated or implied in the text establishing the CG which you read on Galnet or when you sign up for it. Example: "Faction XxX wishes to complete their terraforming project on Myxylplyx 3". The "global reward", in this case, will be the transformation of the space station orbiting Myxylplyx 3 from a "Terraforming" economy to an "Agricultural" one, and everyone, whether they took part in the CG or not, will be able to go there and buy food and sell tractors and fertilizer there. A CG stating "We wish to commence exports of a new rare commodity, Sapyx Mud" will have as the "global reward" a new Rare Good, called Sapyx Mud, available for sale at that station and anyone, whether they participated in the CG or not, can buy that Rare Good there.
Global rewards are not usually implemented straight away - they usually have to wait until the next Thursday server update, or even for the next downloaded update for major changes such as constructing a new space station.
Not every CG has these Global Rewards. The recent Halloween CGs, for example (both the trade and bounty-hunter CGs) had no global rewards - the faction hosting the CG just wanted to throw a big party.
And sometimes, as others have stated above and seem bitter about, the Global Rewards are obscure. I seem to recall there was a CG a few months ago where an anarchy faction wanted stuff smuggled. Smuggling CGs are rarely popular and few people supported this random Indie anarchy faction, so not many people participated and the CG didn't reach anywhere near max tier. Turned out, the (secret) global rewards were for the introduction of cool new weapons, and the failure to reach max tier meant the Large variety of the weapon was withheld from sale.