This is apparently some sort of shelving peg, and the guy is Valentine, the nice man who was helpful enough to explain what the peg was and its function.
When making a shelving unit a way to add the shelves is to drill holes vertically along the interior sides and then squeeze the rounded part of the pegs into the holes. Then you can just rest the shelves along the right angle part of the peg instead of permanently attaching them. In this way you can easily rearrange the shelves or remove them entirely for easier moving of the entire shelving unit.