There are 2 main differences between Maker’s 46 and Maker's Mark. The first is that the 46 is aged longer which mellows it out more while adding more flavor. The second is that seared French oak staves are seeped in the bourbon towards the end of it's additional aging process bringing out some even more flavors. Other than those 2 factors, which are admittedly important, it's the same thing as the original Maker's. Same mash, same barrels, same red winter wheat, same everything.
Maker’s 46 gets it's name from the profile number of the seared French oak staves that are added during the extra aging. The profile number corresponds to the charring rate and temperature development in the staves which can bring out different flavors and characteristics in the wood. Those characteristics can in turn change the flavor of the whisky aged in it which becomes very noticeable when tasting Maker's Mark and Maker’s 46 side by side. The extra aging and the double wood really transforms this whisky into something uniquely tasty.