The general idea of parallax is to use triangulation to measure distances. When looking at an object from two different vantage points, the object appears to shift its position relative to the background. For astronomical applications, the position of Earth shifts by up to 186 million miles (300 million kilometers) as Earth orbits the Sun. So it is possible to view distant objects, such as stars, at two different vantage points. The measure of the amount of apparent change in position of that object is its parallax. Once the parallax is known, it is possible to calculate the distance to that object.
By viewing objects in space from different positions, such as from the Earth and from a space telescope, astronomers can determine their distance. (NASA/JPL-Caltech/T. Pyle (SSQ)