To put it in simple terms, refraction is the process of waves bending when they enter a medium that redirects their form. Bending of waves occurs with both sound and light.
Visual examples of refraction help us to understand the concept of refraction itself. One example could be a toy car going from a wooden floor to a carpet. The toy will slow down and curve in direction because one wheel will hit the carpet first and thus be caused to slow down first, causing an inevitable bend in direction. This is refraction.
Sound in general emanates in all directions, however refraction allows sound to bend where it normally wouldn't travel. A good example of this is a fisherman at the lake in the early morning. The water is cool, however the sun has just begun to heat it up and therefore the air arising from the lake is slightly warmer. Another fisherman calling at him "good morning" from the other side of the lake will sound much closer, due to the fact that sound refraction has occurred as a by product of the air being warmer above the lake, and sound travels faster through warm air. This is a very rare natural occurrence, and also a rare natural example of refraction of sound.