What Is a Zodiac Sign and How Is It Decided?
A sign records which thirty-degree slice of the sky the Sun occupied at the moment you were born. The zodiac belt divides 360 degrees into twelve equal parts, and each part is a sign. When someone says "my sign", this is what they mean: the Sun sign.
It is also why the date ranges drift by a day between years. The Sun moves from one sign to the next by actual angular position, not by the calendar, so the crossing lands on 22 August some years and the 23rd in others. If you were born on a boundary, your sign is not settled until you know the exact time.