What a nakshatra actually is
The zodiac is divided two ways in Vedic astrology: into 12 rashis (signs) of 30° each, and into 27 nakshatras of 13°20′ each. Your nakshatra is simply the one the Moon was sitting in at your birth. Each carries a ruling deity, a symbol, an animal, and a planetary lord, and together these describe temperament at a level a Moon sign cannot — two people with the same Moon sign but different nakshatras often feel quite different from the inside.
How you find yours (and why the time matters)
Because the Moon moves fast, it passes through a nakshatra in roughly 22–26 hours, and each nakshatra is further split into four padas (quarters) of 3°20′. So an accurate birth time is what pins down both your nakshatra and your pada — and the pada matters, because it sets your Navamsa (D9) sign, the chart of marriage and dharma. Without a birth time you can usually narrow it to two or three; with one it is exact.
Why it shapes so much of the chart
Your nakshatra does three quiet but decisive things: it colours your instincts and emotional style through its deity and lord, it fixes the starting planet of your Vimshottari dasha (so it literally sets the timing of your life), and it drives kundali matching, where two people’s nakshatras are compared for compatibility. Knowing your birth star is often where a real chart reading begins.