What is meant by Transition of Image and Sound?

Image - The way in which any two video shots are joined together is called the transition. Transitions are very important, everyone from the camera operator to the editor must have a good understanding of how to make effective transitions. 

In this clip, at 1:48, the shot changes from a dream sequence to the reality of the female character waking from her sleep. A cut transition is used here.

Sound - A transition of sound is done when one song fade out and another fades in, or when a song end for the start of dialogue.

In this clip sound tradition is used at 0:38 when the original score fades out and Life on Mars by David Bowie begins to play. This is when the male character realises something from the past.