Not seen this type of cloud for a while. They’re called Kelvin-Helmholtz wave clouds and are named for two of the godfathers of nineteenth century science.
This form of cloud appears between two layers of air of different densities, travelling at different speeds. If a warm, less dense layer lies above a layer of colder, denser air, and the wind shear across the two layers is strong enough, eddies will develop along the boundary. This gives the cloud an obvious rolling waves appearance, hence their other name “shear-gravity waves”.