A reflection flips a shape over a mirror line. Every point lands on the opposite side of the line, the same distance away.
Reflecting over the x-axis: (x, y) → (x, −y). The y flips sign, x stays the same.
Reflecting over the y-axis: (x, y) → (−x, y). The x flips sign, y stays the same.
The reflected shape is a mirror image — same size and shape, but flipped.
Remember
Over the x-axis → flip the y. Over the y-axis → flip the x. The axis you're reflecting over stays the same, the other coordinate flips.
Real World
A reflection in a still lake — every point above the water appears the same distance below the surface. The waterline is the mirror line.
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