The Ascendant (or "rising sign") is the exact zodiac degree that was rising on the eastern horizon at the precise moment and place of birth. It marks the cusp of the 1st house and is one of the four angles of the chart, along with the Descendant, Midheaven, and Imum Coeli.
Where the Sun describes identity and the Moon describes emotional instinct, the Ascendant describes the interface a person presents to the world — often read as first impressions, physical demeanor, and instinctive reactions to new or unfamiliar situations.
The Ascendant changes roughly every two hours, which makes it the single most time-sensitive point in the entire chart — an accurate birth time, not just date, is required to calculate it correctly. Without a known birth time, the Ascendant (and every house cusp) cannot be reliably determined at all.