The word maria comes from the latin word for the sea, this is why you might also of heard of terms like "the Sea of Tranquility" where the first Apollo astronauts landed. In Latin maria is the plural - seas and mare is the singular sea. It was once believed that a mare was a real sea on the surface of the moon.
Maria can look like seas due to their smooth dark appearance, it is however now known that they are not seas but the name remains the same today.
Image of Mare Tranquillitatis sometime known as "the Sea of Tranquility". Its size is many times that of most of the craters on the moon at over 500km across. Image taken from Clementine mission. Note the horizontal line across the image, this is the equator of the Moon.