With a Lap Dissolve transition, the first shot ends with a fade out, as simultaneously, the second shot fades in. Thus, it gradually overlaps with the first shot as it fades out.

The lap dissolve is thus similar to a fade-out, fade-in. However, the main difference is that the middle sequence of briefly showing a plain-colored screen is left out. That way, the end of the first and the beginning of the second shot are briefly superimposed.

In continuity editing, lap dissolves are mainly used to flag a temporal discontinuity (i.e. the passage of time).