Camera Import: This opens the window for handling video imports from your camcorder or webcam. To import video files instead, use File Menu > Import > Movies.
Swap Events and Projects: This button just changes the position of the Project Library and Event Library panels. "Traditional" video editing software interfaces usually have the raw clips in the top left and the editing area in the bottom right.
Thumbnail Size: This slider controls the size of the Event and Project thumbnails. If you want see more of the overall project, slide left to zoom out. If you want to see more details of a specific section, slide right to zoom in.
Here are the buttons in the center of the iMovie toolbar