Thumbnails are a still image from the camera footage for a specified moment in time. To reveal thumbnails press on the Thumbnails button from the bottom toolbar. The time interval between thumbnails depends on the scale of the timeline.
Hover mouse pointer over the timeline (blue line in the bottom of the screen) to change the time range for which thumbnails are shown. Timestamp at the bottom of each thumbnail shows the exact time when this still image was captured and represents the actual snapshot from the camera. Click on any image from the thumbnail line to start playback from the specified moment in time. The time interval between thumbnails depends on the scale of the timeline, you can use your mouse wheel while pointing at the timeline to change the scale factor of the timeline or use a popup menu from the bottom toolbar.