A parallax is a phenomenon caused by observing an object from two different observation points, e.g. two eyes. If you hold up a finger and close your right eye and then close your left eye, it will look as if the finger moves relative to the background.
In order to get a parallax when viewing a far away object, lika a star, the two observation points must not be close to each other.
The first star parallax was observed in 1838 by Friedrich Bessel. He found that the star 61 Cygni had a parallax of 0.314 arcseconds, i.e. that the angle subtended by the star as the earth orbits the sun is 0.314/3600 degrees.
The unit for measuring the distance to stars is parsec, the parallax of one arcsecond.
The Hipparcos satellite
image from: NASA
Hipparcos=founder of trigonometry
by Malin Christersson under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 Sweden License