This small metal piece appears to be a watch spring bar, also called a watch band pin or spring pin. It is the tiny spring-loaded rod that connects a watch strap or bracelet to the main body of the watch, often called the watch case or “face.” The bar sits between the watch’s lugs, and its ends compress inward so the strap can be installed or removed.
In simple terms, its job is very important: it holds the watch strap securely in place. Without this small part, the band could not stay attached to the watch. It also makes it easier to replace straps, which is why it remains a standard part in most modern wristwatches. Even though it is tiny, it carries the weight of the watch and helps prevent the watch from slipping off the wrist.