The filmstrip, when enabled, is shown at the bottom of the screen (except in the lighttable view, where it is replaced with the timeline module) and displays the images from the collection that is currently selected in the lighttable view. You can navigate along the filmstrip by scrolling with the mouse wheel.
The filmstrip allows you to interact with images while you are not in the lighttable view. For example, while developing an image in darkroom mode, you can switch to another image to by clicking its thumbnail in the filmstrip. You can also rate and color-classify the images as you do in lighttable, as well as copy & paste the history stack using keyboard shortcuts.
See the filmstrip module documentation for more information.