Keck Clips are used for holding the connecting joints of bongs together (well, not just bongs, but you know what I mean). One is usually used to secure the downtube or diffuser in the female joint of the bong, another Keck Clip can be used to secure an additional piece such as a Carbon Filter Adaptor.
If your bong has a carb-hole which you prefer to use instead of the separate slide bowl, then you can use another Keck Clip to secure the bowl more permanently.
If you are wondering why the hell they are called Keck Clips, it’s because they are named after Hermann Keck who patented them in 1984. They are usually (but not definitely) colour-coded in size order, with the blue Keck Clips suitable for 18.8 joints and the yellow Keck Clips for 14.5 joints.