The Midheaven (Medium Coeli, or "MC") is the point of the ecliptic directly overhead — the highest point the Sun reaches at true noon from that birth location — and marks the cusp of the 10th house. It's one of the four angles of the chart, along with the Ascendant, Descendant, and Imum Coeli.
Where the Ascendant describes how someone instinctively meets the world, the Midheaven describes what they're building outward and upward: career direction, public role, professional reputation, and the legacy a person is recognized for. Planets near the Midheaven tend to be highly visible — traits that show up in someone's public life rather than staying private.
Like the Ascendant, the Midheaven depends on an accurate birth time, since it's derived from the exact moment and location of birth rather than the date alone.