A kundli (or janam kundli) is a diagram of the sky at the moment and place of your birth, as interpreted in Vedic astrology. To draw one, an astrologer needs three things: your date of birth, your time of birth, and your place of birth.
The first building block is the lagna, or ascendant — the zodiac sign rising on the eastern horizon at your birth. Because the ascendant changes roughly every two hours, an accurate birth time matters. This is also why birth-time rectification exists as a separate practice.
The second building block is the twelve bhavas, or houses. Each house is traditionally associated with areas of life — for example, the first with self and body, the fourth with home, the seventh with partnership, and the tenth with work and public life.
The third building block is the grahas — the Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, and the lunar nodes Rahu and Ketu. Interpretation traditionally considers which sign and house each graha occupies and how they relate to each other.
Charts are drawn in different regional styles — North Indian charts are diamond-shaped, while South Indian charts use a fixed square grid — but the underlying information is the same.