A parabola is the set of points that is equal distance from a line called the directrix (blue line) and a point called the focus (red dot on y-axis). Move the red vertical line and a parabola will be traced out where the distance from the directrix to the parabola is the same as from the focus to the parabola. You can also move the focus and move (even tilt) the directrix to see how the shape of the parabola is effected.