Hi Floyd, Factory rebuilt, or refurbished means the pistol went back to the Glock factory, and was thoroughly inspected and anything worn or out dated was replaced by Glock at the factory. (generally, the guns appear to have brand-new internals).
They will stamp the inside of the "dust cover" portion of the frame with a "7" to indicate a factory rebuild gun.
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.
Stay safe & have a great day!

Glock has gone through the gun and replace any worn parts they do not repair cosmetic faults

Rebuilt means, just like a rebuilt motor, all internals have been replaced with new, the slide & frame would most likely be the only original parts that weren't replaced.

Hi Floyd and welcome. Depends on the condition of the pistol before the rebuild. Generally they clean, lube, and replace any worn, damaged parts, or parts that require upgrading such as springs and smaller internal parts when a gun is sent back to them. Glock replaces any worn part(s) and refinished the pistol to good as new. Cheers and stay safe!