These low profile devices fit between two 0.5mm FFCs (flat flexible cables), connecting them together mechanically and electrically. To fit a cable, pull the black section out of the white section as shown in some of the listing images. Insert an appropriate cable, observing the correct orientation of the electrical contacts, and push the black section back into the white section to secure the cable. At just 2.5 mm thick, they are the most compact solution for connecting together two FFCs outside of (low temperature) soldering.
eBay