The axis of symmetry in a parabola is the axis that passes through its vertex in such a way that if we folded the right side over the left side, both sides would appear joined.** Let's see it in an illustration:**

To find the axis of symmetry, we must locate the value of $X$ of the vertex of the parabola or do it through the parabola's vertex formula or with the help of two symmetric points on the parabola.