The pine tree in the middle is taken from Maine’s ensign and is a prominent symbol of Maine.
The pine green stripe represents Maine’s forest.
The star in the top left is taken from Maine’s old flag and represents the North Star, which itself symbolizes Maine’s motto, “Dirigo” (meaning “I lead”).
The buff is taken from Maine’s old flag.
The red symbolizes the state’s presence in New England.
Symbolism:
EDIT: Fixed an error.