A reflection is a "flip" of an object over a line. Let's look at two very common reflections: a horizontal reflection and a vertical reflection.
How to draw the reflection of a plane figure
A reflection in a line produces a mirror image in which corresponding points on the original shape are always the same distance from the mirror line.
The reflected image has the same size as the original figure, but with a reverse orientation.
Examples of transformation geometry in the coordinate plane...
A(0, 6) A'(0, 6)
B(5, 6) B'(-5, 6)
C(4, 2) C'(-4, 2)
D(2, 2) D'(-2, 2)