Put your index finger at arms length in front of your face. Now close your left eye and note the position of your finger relative to other objects in your room in front of you. Now open your left eye and close your right eye. Note that your finger has shifted its position relative to the background objects in your room. This is the parallax effect.
By photographing a distant object, first from one side of Earth's orbit, then 6 months later from the other side, your vantage point has shifted, not by the distance between your eyes, but the entire diameter of the Earth's orbit some 186 million miles. If the objects is near enough, its position relative to more distant background stars will change by a measurable amount. If the star is 3.24 light years away, it will shift by exactly 1 second of arc. This is an angular distance equal to 1/3600 of a degree. There are 60 arcminutes in a degree, and 60 arcseconds per arc minute.
Astronomers have used this technique since the time of Tycho Brahe in the 1500's and is the lowest rung on the entire distance ladder astronomers use to determine how far away stars, galaxies and quasars are in the universe.
The parallax effect is also used on the Earth in the design of range finders.