What does axis of symmetry of a parabola mean

If you had a piece of paper in the shape of the parabola, the axis of symmetry

is the line on the paper where you could fold it, and the two edges would exactly

come together. It's the line right down the middle of the parabola.

The axis of symmetry of a parabola is a straight line which divide graph of parabola in two symetric branches.