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. The analogy with stellar parallax is that your two eyes are two positions of the Earth separated by 6 months, your finger is a nearby star and the vertical edge is a distant star, used as a reference point. This is the immense explosion which takes place when the core of a massive star collapses under its own gravity. A white dwarf is what's left of the core of a red giant when it has lost its outer layers as nebulae .
White dwarfs are extremely dense, with electrons packed very closely together in their core.