iMovie displays your video footage in the Project browser (where you build projects) and Event browser (where your source video is displayed) as filmstrips, which are connected series of “thumbnail” images. Unlike in an actual movie reel, where each thumbnail is a single frame of the movie, in iMovie, each thumbnail can represent several seconds of video encompassing hundreds of video frames (individual images).
You can use the thumbnail slider (shown below) to expand filmstrips to see more thumbnails in each clip, or contract them to see fewer, depending on how you like to work, how long your project is, and so on. The number that appears to the right of the slider indicates how many seconds of video are represented by each thumbnail image.
When you add video from the Event browser to a project, you can add one or more entire clips, or you can select a range of frames within a clip to add. When you add clips or a range of frames to your project, they appear as their own series of thumbnails, unconnected from other clips. This lets you fine-tune the clips individually, adding special effects or sound effects to them, for example, or inserting transitions between them.