Parallax is a method of using two points of observation to measure the distance to an object by observing how it appears to move against a background.
In astronomy, the distances to other stars is too great to measure using two objects on the Earth's surface. Lucky for us, the Earth itself moves. If we made two observations of the same star on opposite sides of the Earth's orbit, we would have a separation of 2 astronomical units, or AU.
One AU is the average distance from the Sun to the Earth.This is enough to get a noticeable angle, α, between the star's two apparent locations. In the image above, we can see that by cutting α in half, we get a right triangle where one leg is the distance between the sun and the other star. Lets let 1/2 α=p. We can use tan p to find the distance to that star.
tan p= 1AU/d
Since the star will be very far away, we can make the assumption that tan p is about equal to p. That simplifies our parallax formula to;
p=1AU/d, or in other words, d=1AU/p
Astronomical units are not the most convenient units to work with, though, so instead we define a parsec to be the distance to a star that shows 1 arc-second of parallax angle. Our formula then becomes;
d=1p parsecs
Where p is measured in arc-seconds. 1 parsec is about 3.3 light years.
There is more information of Parallax Formula in this link .