When you describe something as main, you mean it's the most important, well-known, or biggest of its kind. The main branch of the post office in a city is usually the largest and busiest.
The main theme of your English paper is your thesis, the basic, fundamental thing you're trying to express in your writing. The main road in town is the most busy and populated — in fact, in many places this road is called "Main Street." This adjective describes things that stand out as the most important or prominent in their category. In the thirteenth century, it meant "large or strong," and came from the Old English mægen, "power, strength, or force."