Click on the transition tab on the bottom left corner.
After choosing a transition, click on it and drag it before or after the video clip you are working with.
Kissing arrows: the symbols that appear when a transition has been chosen.
The same steps apply for adding a title. However, there is one more option offered by iMovie regarding titles. Along with being able to drag titles before and after videos, titles can also be dragged above videos in order for the text to appear on the actual video itself.