To add text on top of your clip, click on Titles in the Content Library (bottom left corner in iMovie). You can preview the different types of titles (each titles has different editing restrictions). Drag the desired style on top of your clip. Adjust the font style, size, colour and alignment, if available. Adjust the duration and start times of the title by dragging the ends, just like when you’re trimming a clip.
For a full title slide at the beginning, end or middle of your video, drag a title style to the beginning, end, or between clips on the timeline.