The act of purchasing a video game or software before it's actually released; often to gain some slight reward, easter egg or advantage when eventually playing the game itself.
The term was coined by the software vendors, and literally represents an impossible act, since what you are doing is actually regular ordering, with real money. The correct definition of what you are doing is "post getting".
"I had a spare $50 lying around, so I pre-ordered Moons Over Mars so my character would have the g-string of death. Had I waited for 3 weeks post-release; I would have only paid 19.50"
To order something before you want it, as distinct from after you want it.
"Pre-order your Christmas turkey now. Don't wait until Boxing Day."