What the Midheaven is
The Midheaven, or MC (from the Latin medium coeli, "the middle of the sky"), is the point where the ecliptic, the Sun's path through the zodiac, crosses the meridian overhead at the moment of birth. It is the highest point of the birth chart, and together with the rising sign (the Ascendant), the setting point and the IC below it, it forms the four angles that frame the chart. The Midheaven turns through all twelve signs every day, so it depends on the exact time and place of birth.
What your Midheaven sign means
Astrologers read the Midheaven as your public life: your career and calling, what you want to achieve and how the world sees you at your best. Where your Sun sign describes who you are and your rising sign how you come across, the Midheaven describes the direction you aim for. Its sign colors that direction: a Capricorn Midheaven is read as a steady climb toward authority, a Gemini Midheaven as a career in words and ideas. These are readings from tradition, not predictions.
Its ruler, and planets on it
The planet that rules your Midheaven sign is read as the key to how you pursue your calling, and where it sits in your chart, by sign and house, adds detail. Planets within a few degrees of the Midheaven are read as especially prominent in public life: the Sun there for a visible role, Saturn for hard-won authority, Venus for work with art or people. The calculator names your Midheaven's ruler and any of the ten planets within 8° of it.
The Midheaven, the 10th house and the IC
In most house systems, including Placidus (the default here), Porphyry and Koch, the Midheaven is the cusp of the 10th house, the house of career and reputation. In Equal and Whole Sign houses the houses are counted from the rising sign, so the Midheaven usually falls in the 9th, 10th or 11th house. Exactly opposite it is the IC, the lowest point of the chart, read for home and roots: the Midheaven and IC form the chart's vertical axis, from private foundations to public achievement.
Why you need a birth time
The Midheaven moves through the whole zodiac in a day. It spends between an hour and 51 minutes and two hours and 9 minutes in each sign, the same at any latitude, because it follows the turning of the sky rather than the horizon. So the birth time matters, and a time that is off by an hour can move it into the next sign. Without a time, the table in the result lists which sign was on the Midheaven at each part of your birth day, so you can see the candidates.
Your MC lines
In astrocartography the Midheaven becomes a line on the map: each planet's MC line runs through the places where that planet was at the top of the sky at your birth. They are read as lines of career, visibility and reputation, and the lines for career guide explains how to use them. The astrocartography map draws all of yours.