The Lesson
A reflection flips a shape. A reflection is a flip of a shape about a line (called the line of reflection). A reflection is a type of transformation.A Real Example of a Reflection
It is easier to understand reflection with an example. The diagram below shows a reflection of a shape. The shape before the reflection (called the object) is in light blue. The shape after the reflection (called the image) is in dark blue:In this example, the shape has reflected in a line of reflection (shown as a red dashed line) on the yaxis.
Properties of a Reflection

Each point in the image is the same perpendicular distance from the line of reflection as the corresponding point in the object.
Point A' is the reflection of point A. They are both 3 squares away from the line of reflection. Point B' is the reflection of point B. They are both 1 square away from the line of reflection.  The image is the same size as the object.
 If you were to fold the diagram along the line of reflection, the image would fit exactly over the object.