What a rashi actually is
Your rashi is the zodiac sign the Moon occupied at the moment you were born. Vedic astrology reads the Moon, not the Sun, as the significator of the mind, and almost everything written as a horoscope in India is keyed to the Moon sign. This is why the sign you know from a newspaper column — which is usually the Sun sign — often does not match the one an astrologer names.
Why the date alone is not enough
The Moon moves through the whole zodiac in roughly twenty-seven and a half days, spending about two and a quarter days in each sign. People born on the same date in different years almost never share a rashi, and a birth near a sign change can fall either side of it depending on the hour. Entering the time and place is what makes the answer yours rather than approximately right.
What to do with it
Once you know your rashi you can read the daily, weekly, monthly and yearly rashifal written for it, and check whether Saturn is currently transiting over it — the seven-and-a-half year period called sade sati is defined entirely by where your Moon sits.
Common questions
Is my rashi the same as my zodiac sign?
Usually not. Indian astrology uses the sidereal zodiac and reads the Moon, while a newspaper horoscope generally uses the tropical zodiac and the Sun. The two differ by around twenty-four degrees, so your rashi is often the sign before the one you already knew.
What if I do not know my birth time?
The rashi is reasonably robust: the Moon stays in one sign for about two and a quarter days, so an uncertain hour usually does not change it. A birth close to a sign change is the exception, and there the time decides the answer.
Which sign should I read my horoscope for?
Read the Moon sign. That is the sign Indian rashifal is written for.