The coat of arms consists of a shield that depicts an Algonquian Native American with a bow and arrow and the arrow pointed downward signifying peace. Above the shield is the state military crest: a bent arm holding a broadsword aloft. The sword has its blade up, to remind that it was through the American Revolution that independence was won.
The motto, Ense petit placidam sub libertate quietem, is Latin for, "By the sword we seek peace, but peace only under liberty."
However the literal translation is "she seeks with the sword a quiet peace under liberty" from a longer phrase, "this hand, hostile to tyrants, seeks with the sword a quiet peace under liberty."
The motto comes from Englishman, Algernon Sidney, who penned it around 1659. Sidney was an opponent of Charles II and was executed for treason. The coat of arms is found on the current Great Seal and on the flag of the Commonwealth of Massachusetts.
17 other states have officially adopted coats of arms as well. You can see them here.