Transitions are effects you can add to help make one clip within your video go smoothly into the next.
Transitions can be adding using the Transitions panel. Select the clip you want to add a transition to and open the Transitions panel. Select the transition effect you want to add.
By default it will add a transition to both the start and end of your clip. If you want to only add it to the start or the end of a clip, select the transition effect you want from the Transitions panel and drag it to the start or end of the clip.
You can adjust the length of a transition using the Duration slider in the Transitions panel.