Suppose you have two media that meet at an interface. Sound starts in medium 1 and hits the interface.
The wave is reflected off the interface if it stays on the same side and stays in medium 1.
The wave is transmitted if it crosses the interface and enters medium 2. Typically the wave changes speed and direction as it crosses because the two media have different properties. Refraction refers to the change in direction as the wave crosses the interface.
Often part of a wave will be reflected and part transmitted/refracted.
If you are talking about seismic waves, you may be talking about a non-uniform medium, where the properties change continuously from one part to another.
In this case, you can see refraction as the wave passes from one part of the medium to another. Instead of an abrupt change in direction at a boundary, the wave may follow a curved path in a single medium.
In all cases, refraction is a change in wave speed and direction caused by changes in the properties of the medium in which it travels.