Detail Texturing allows you to give the illusion of more detail in a Texture by bringing in a highly repeated Diffuse and Normal map combination which layers over the original Diffuse and Normal for an object.
This gives the illusion of greater detail at close range than normally would be possible.
Here is an example of Detail Texturing in action.
On the Left Hand side, labeled with a number 1, you can see a Material that does make use of Detail Texturing.
On the Right Hand side, labeled with a number 2, you can see a Material that does not make use of Detail Texturing.
Notice how in the image on the left, 1, there looks to be more detail from the Normal map and that detail looks much sharper than the image on the right.