That doesn't mean I'm not going to not not explain it to you.
Unlike the above sentence, this product doesn't require much thought. Some of us still remember the good ol days where BNC was King of the connections and only the absolute best monitors (CRT screen and all) had them. BNC (Bayonet Neill–Concelman, named after it's inventors) was originally designed for military applications but quickly became a favourite in broadcast radio and television due to its superior connection and minimal wave reflection/loss. It was even used to transfer data on some of the earliest computer networks such as ARCnet and the IBM PC Network.
This fine example of a coupler features Female BNC connectors on both ends so you can extend your BNC cables by joining them together.
Fun Fact: Sometimes, incorrectly, the acronym for BNC is referred to as coming from "Bristish Navy Connector"