I suggest taking the ANA Course on the Modern Minting Process and Errors and Varieties.
That is where I got this knowledge. I will present a brief summary.
That means a die which was hubbed (the design impressed on it from a hub),
and then hubbed again but the designs did not match up exactly.
That leaves two or more "images" on the die. The original image is called the secondary image, because the newer image is higher in relief.
There were many reasons for the second hubbing, from the multiple-squeeze hubbing process, to replenish the design, or to change the date and use the die for another year.
Since the mid 1990's, the US Mint uses the "single squeeze method", so there is little possibility of new hub doubled dies. Many of the "doubled die" coins you see (struck after 1997) for sale are merely strike doubled (or MDD) coins and not worth a premium. This is especially true of the so called Doubled Die state quarter.
There are many classes of hub doubling: Offset, Rotated, Pivoted, design differences, and more.
In the case of a Doubled Die, the second image is "added" to the
In the case of Strike Doubling, the second image "smashes" the first image.
This is a true Doubled Die coin (1995 DDO). [ Coin from Albert Sims ]
The serifs are split, and sharp (relatively).
This is Strike Doubling - not a Doubled Die coin (1969-S). [ Coin from Albert Sims ]
(Also known as Machine Damaged Doubling.)
The serifs are rounded out (smashed).
This is an Inside Abraded Die Doubling - Not Hub Doubled.
The hub's devices/lettering has flattened and widened from use.
This is an Outside Abraded Die Doubling - Not Hub Doubled.
"Poor Man's Doubled Die"
The hub's devices/lettering has flattened and widened from use, and moved away from the center of the design.