What is MTF?

The Basics:

MTF stands for Modulation Transfer Functions is a lens performance evaluation method used to determine the contrast reproduction ratio, or sharpness, of a lens.

A Little More Detail: 

MTF charts are also used to indicate the resolving power of a lens and there are usually two graphs, one for 10 lines/mm and one for 30 lines/mm. Owners of high megapixel cameras tend to look at the 30 lines/mm graph.
At a quick glance, the MTF chart also indicates sharpness from centre to edge.