The focus point of the parabola lies on the axis of symmetry.  It is a fixed point in relation to the parabola.  In diagram 1 the focus point is identified in red.  Remember that the vertex point of the parabola is located where the axis of symmetry and the parabola intersect.

The directrix is a fixed line in relation to the parabola.  In diagram 2 the directrix is identified in blue.

The Focus point F and the Directrix line D are located in such a way that any point P of the parabola is the same distance from the focus point F as it is from the directrix line D.  For all of the points on the parabola, each one is the same distance from the focus point as it is from the directrix line.  In diagram 3 both of these distances are represented by a blue line and are identified as "a" on both lines.  The line representing the length from the parabola to the directrix is always perpendicular to the directrix line.   Note that as the point P moves on the magenta parabola, the blue lines move and change length, however as their lengths change, the two lengths are always equal.

In diagram 4 the point P is on the Vertex point of the parabola.  Notice that the vertex point is also the midpoint between the focus point F and the directrix point D.  So if we know the coordinates of the focus F and the directrix point D, we can use the midpoint formula to locate the coordinates of the Vertex point.