This is the apparent shift in the position of a star if it is viewed from the Earth at different times throughout the year. To get an idea of the effect, hold a finger in front of you and using one eye, line it up with a vertical feature in front of you, like the edge of a door. Keep your finger where it is and look at it with your other eye: the change in viewpoint makes it look like your finger has changed position with respect to the vertical background object. The analogy with stellar parallax is that your two eyes are two positions of the Earth separated by six months, your finger is a nearby star and the vertical edge is a distant star, used as a reference point.
The change in a star's position, during a year, due to the Earth's motion around the Sun. More specifically, the semi-major axis of the parallactic ellipse produced by this motion.