Main means the largest or most important of something. For example a company might have their main office, and many towns have a main street.
Two people are married if they have been through a wedding ceremony together and have formally become a couple.
Something is mature if it has fully reached adulthood. A fruit or nut is said to be mature if it is fully ripe. Other foods like wine and cheese where you need to wait a long time for the flavor to change are said to be mature when they are ready to eat or drink.
Something is messy if the things in or on it are arranged randomly, so they don't look very nice. The opposite of messy is neat.
Military means anything relating to the armed forces, particularly the army.
Something is miniature if it is a very small copy of something else. A miniature can either be a model of something, like a miniature car, or a word for a very small animal, such as a miniature horse.