Hi John, it is rebuilt at the Glock factory. The following is done to a "Factory Rebuilt" Glock handgun:
1) Inspect and clean the gun
2) Replace any worn or outdated parts (generally, the guns appear to have brand-new internals). Frames lacking a relief cut will have it added.
3) Replace dead tritium sights with current-manufacture polymer sights.
4) Stamp the inside of the "dust cover" portion of the frame with a "7" to indicate a factory rebuild gun.
5) The gun will be shipped in a current-production padded case with two magazines, a cable lock, cleaning rod/brush, instruction packet, and loading tool. The label on the box will be stamped "Rebuilt" with a date listed. The gun will have a silver label affixed to the trigger guard indicating it is a factory rebuilt gun.
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In most cases, rebuilt Glocks started off as floor or display models, maybe even a sample run that was made to show off the firearm. Rarely will any of them have any round count, and certainly not a high round count.
When you purchase a factory refurbished model, you are basically getting a brand-new firearm with its full life in front of it.
Stay safe & have a great day!

It means the gun was gone over by an armorer and any worn parts replaced. Cleaned lubed and test fired.