The vertex of a parabola occurs where the parabola and the line of symmetry of the parabola intersect.
When the parabola is vertical (and the line of symmetry is vertical), the vertex is the lowest point on the parabola when it opens up, and is the highest point on the parabola when it opens down.
When a quadratic function is graphed in the coordinate plane, the resulting parabola is vertical. The ordinate of the vertex is a minimum value when the parabola opens up, and a maximum value when the parabola opens down.
