A rigid motion is any motion that doesn't deform the original shape - it's a motion that preserves the lengths and angles of a shape without stretching, squishing, or bending anything. The best way to think about rigid motions is to imagine holding something solid in your hand, like a smartphone. How can you move it around? You can rotate it around in your hand, changing its orientation; you can move your hand around through space, changing the smartphone's position. While it may not seem like a motion, you can also hold it up to a mirror, reflecting it in some way. These three transformations, rotation, translation (shifting position), and reflection, are the three primary rigid motions.