It is crucial to be able to measure the distances to stars if we are to derive intrinsic luminosities. There are several methods for measuring distances to stars, but the most reliable by far (when it can be applied) is Trigonometric Parallax.
Hold a finger up in front of your nose, close one eye and note where the finger appears on the back wall. Now close the other eye (and open the one that was closed before) and note that the finger appears to move on the back wall. This is the parallax effect - the apparent motion of a nearby object compared to distance background objects because the change in viewing angle.
The asterick is a nearby star which is apparently moving back and forth every year compared to the more distance background stars. Note the star at the lower right which has a proper motion.
The distance to the star is inversely proportional to the parallax angle (which is usually indicated by the symbol ).
What is an arcsecond ( )?
This is the angular size of a dime seen from 2 miles.
So, a star at 80 pc has a parallax angle of only arcsec.