How are parsecs used in astronomy?
A parsec is used to measure the distance to a star, or to celestial bodies in the sky from where we are on earth.
As shown above, the parallax angle, p, is clearly a measure of the distance to a star. The larger the angle, the closer the star. In fact, the key measure of distances in space is defined as the reciprocal (a pronoun or verb expressing mutual action or relationship) of the parallax angle measured in arc seconds (that is one 3 thousand six hundredth of a degree). Stated simply, a star with a parallax angle of 1 arcsec is said to be at a distance of one parsec (1 pc).
If you have a further interest in this topic, look at the math behind it in this video from youtube.