What do the in m and candy stand for?

You're likely thinking of the candy brand M&M's.

The "M&M" stands for Mars & Murrie, the two companies that created the candy.

* Forrest Mars was the founder of Mars, Inc.

* Bruce Murrie was a member of the Hershey family and the head of the company that produced the chocolate used in the M&M's.

They partnered to create the candy and the name reflects their collaboration.