Snell's law is about waves in general and light rays in particular. When a light ray from a medium 1 falls on an interface between the two different media, a part of it gets reflected back and a part of it refracted (transmitted) into the 2nd medium. Snells' law has specified the geometry of the bending of light at the interface of the two mediums.
Let i is the angle of incidence made by the light ray or wave with the normal (perpendicular) drawn at the point of incidence on the boundary between the media.
Let r be the angle that transmitted ray in the 2nd medium, makes with the normal at the point of incidence.
Sin i / Sin r = constant = refractive index
of the medium 2 with respect to the medium 1.
Ratio of sine of angles of incidence and refraction is inversely proportional to refractive
indices of the respective media.
This implies that when a light ray enters from a thinner medium (air) to water or glass (thicker), the ray bends towards the normal at the surface. So angle of refraction is less than angle of incidence.
This law also has further implications related to the wave characteristics like wavelength, speed etc.