I am an optical engineer and photographer, but my specialty is not imaging optics. I learned quite a bit from this post about reading lens manufacturer's MTF charts, but MTF itself was not really defined.
In a line pattern, modulation is the ratio min/max of brightness. An imaging lens transfers the object line pattern to its image line pattern with some loss of modulation. Thus "Modulation Transfer.
Modern lens design software can produce theoretical MTF charts for free with a keystroke. Theoretical MTF is useful for comparison, but as-built MTF can never be as good as theoretical. MTF is degraded by lens manufacturing tolerances, assembly tolerances and AR coating performance. It also depends on focus and zoom position.
P.S. I am also a mathematician, which is a requirement to pass LULU's Turing test.