The parsec is a unit of measurement that comes about when you measure how much the position of the nearby star seems to shift as the Earth orbits the Sun. If you put your thumb at arm's length and alternately open and close your left and right eyes, you will see your thumb shift position relative to objects in the background of your room. If you photograph a star at two times of the year, 6 months apart when the Earth is on opposite sides of the Sun, the baseline distance is 2 x 93 million miles. At a distance of 1 parsec, this parallax shift at opposite sides of the orbit is exactly 2 seconds of arc. At 5 parsecs, this shift is 2/5 of an arc second and so on. Technically, at a distance of 1 parsec, a distance of 93 million miles ( the Earth orbit radius) subtends exactly 1 second of arc. There may not be any stars at this distance, but that doesn't matter. The nearest star is Proxima Centauri at 4.2 light years or 1.29 parsecs.