Your rashi is the sign the Moon was in when you were born, and in Indian astrology it is the sign that matters most from day to day. Enter your date, time and place of birth above and you will see which of the twelve is yours. It is free and needs no account.
Magazine horoscopes sort people by the Sun. Indian tradition sorts them by the Moon, because the Moon stands for the mind and how you feel. So when a priest or an elder in your family asks your rashi, this is the sign they mean.
It is also the starting point for much of what you read about yourself. Your rashifal, your Sade Sati and your transit readings are all counted from this sign.
Your moon sign and your sun sign are two different placements, so they often differ. There is a second gap as well: Vedic astrology measures the zodiac against the stars, while popular Western astrology ties it to the seasons, and the two sit roughly 24 degrees apart.
The Moon crosses a sign in about two and a quarter days. If it changed signs on your birthday, your time of birth decides which side you fall on.
Your sun sign shows where the Sun was at birth and your moon sign shows where the Moon was. The Sun stays in a sign for a month, so everyone born that month shares it. The Moon moves on every two days or so, which makes it far more personal.
An approximate time is usually enough, because the Moon stays in one sign for over two days. It only becomes important if the Moon changed signs close to your birth, so give the best time you have.
A few things can explain it: a different ayanamsa, a birth time that was rounded, or a rashi remembered from the name chosen for you. This calculator uses the Lahiri ayanamsa, so start by checking the time you entered.