Weyl invariance is a property of certain physical theories that remain unchanged under a specific type of transformation called scale transformations. In these transformations, the lengths of objects can be stretched or shrunk uniformly, but the overall structure of the theory stays the same. This concept is particularly important in theoretical physics and string theory, where it helps describe how different physical phenomena can be related through scaling. In a Weyl-invariant theory, quantities like energy and momentum can change when the scale changes, but the fundamental relationships between them do not. This invariance can lead to insights about the underlying symmetries of quantum field theories and can help in understanding the behavior of gravity and quantum mechanics at different scales.