Parallax is the angle between the apparent location of an object as seen from two different places. It is used to measure their distance to stars. If a very small angle can be measured accurately, a distance to a nearby star can be determined.
For measuring distances to a star, the parallax used is the angle between viewing the star from Earth at two times, half a year apart, when the positions of the Earth differ by 2 AU (the Baseline). The angle cited as parallax is typically half this, the angle from two positions 1 AU apart, e.g., from the Sun and the Earth. A Parsec is the distance of a star with this (1 AU) parallax angle of 1 arcsecond. Typical capabilities of telescopes:
Secular Parallax is using the Sun's motion to gain a longer baseline, but the fact that the target star also has such a motion limits the information that can be gained.
Cosmic Distance Ladder
Stellar Distance Determination
Stellar Luminosity Determination
Stellar Parameter Determination
Stellar Radius Determination