What is ML and where is it used?

ML could stand for "Markup Language" or "Machine Language".

Markup Language
The most commonly known "Markup Languages" are HTML (HyperText Markup Language), XHTML (eXtensible Hypertext Markup Language), XML (eXtensible Markup Language). "Markup Language" is a system used for annotating text.

Machine Language
Machine Language is the most primitive programming language as it is the lowest level representation of a compiled computer program.