The Midheaven is the point overhead when you were born. If you were born at noon, your Sun would be near the Midheaven (abbreviated as Mc). The zodiac sign of your Midheaven is located at the top of the line that separates the 9th and 10th houses.
The zodiac sign on your Midheaven describes the outward expression of your individuality and ego. It provides information on career, public image, and how the world perceives you.
In esoteric (spiritual) astrology, the Midheaven is considered to represent something you need to take to the world in this life. It's the highest point you can reach in the worldly sense.