NOTE: If you go on reading this post, you will never unsee the real meaning of the M again
I'm talking about this logo here (sometimes it comes with a red background)
It's very simple. The designer had in mind to subliminally remind potential customers of something no-one can resist subconsciously. Everyone is attracted to that, even if you won't admit or realize it. The McDonald's "M" whenever you see it somewhere on a restaurant even if you're far away is supposed to trigger the association in you that you will get served in the most intimate and satisfying way possible.
That's why the M purposely has been designed to have the shape of this:
That's what this M stands for. Irresistible, isn't it?