What Are Reflections, in Geometry?

Reflections flip a figure over a line

In Geometry, reflection basically just means flipping shapes or figures over a line. This line is totally imaginary (which is to say, it might not be visible), and can overlap the shape—or not.

In the image below, the shape that started out with its top left corner at $(-1.25, 1)$ is reflected over a line at $x = -0.5$:

If shapes have reflectional symmetry, it means that they have at least one line of symmetry that they can be flipped over and still look exactly the same.