Stellar parallax is the apparent shift in position of a nearby star against the background of more distant stars as observed from different points in Earth's orbit around the Sun. This effect occurs because as Earth moves, the angle from which we view the star changes, creating a triangle with the star and the two observation points. Astronomers measure this angle, known as the parallax angle, to calculate the distance to the star. The greater the parallax, the closer the star is to Earth. This method is crucial for determining distances in the Milky Way and has helped establish the scale of the universe.