A reflection is a transformation that represents a turn of a figure. Figures can be reflected in a point, a line, or a plane. When a figure is reflected on a line or at a point, the image is congruent to the preimage.
When we make a reflection on the x axis, all the values of the x axis will remain static and the values of the Y axis will change sign:
In the image we can see the triangle reflected on the x axis, the new triangle has vertices DEF