Basically, it's a person bowing! The two M's are the hands of the person, the _'s are the person's eyes, and the ()'s are the sides of the person's head.
(n) A Japanese emoticon used to convey the act of bowing to another. This usually means you admire something of them, or wish to show respect. In some cultures, bowing is also a way of greeting one another (however, if you use this emoticon to greet someone who is not from a culture of bowing, then you will most likely confuse them).
{Example 1}
Sensei (teacher): Goodmorning, pupil.
Student: m(_ _)m
{Example 2}
1: I just beat Halo2 in like an hour!!
Michaila&marshawn...the first letter of each name...Michaila and marshawn went out, then broke up...Michaila still likes marshawn, but doesn't know if he still likes her