Huygens' Principle is a geometric method, dating back to 1678, or determining where a wave goes if you know where it is now.
All points on a given wave front act as point sources of spherical waves called wavelets. The wavelets travel through the medium at the wave speed. The new position of the wave front is the surface tangent to the wavelets.
Christian Huygens used this idea to show that reflection and refraction could be explained using the wave theory of light. It can also be used to explain the diffraction and interference of light.