A transition is an effect that stitches or joins one clip to another. Transitions are used in various ways, such as to tell the audience that there has been a change in scenes or a jump in time. Avoid using flashy or obvious transitions as they are distracting to the audience. The most effective and widely used video transition is Crossfade, which fades one clip out while simultaneously fading the next clip in.

Since transitions work by utilizing the last and first few second(s) of clips to create an animation, they will not work properly between clips that have no more frames beyond their end-points. When the clips are at their max length, theyre marked with a small white triangle on their start- and end-points. 

Applying a transition to a clip at max length will result in an error message and a poorly applied transition. If this happens, try shortening the clips.