A Dark Launch is a software deployment practice where new features and fixes are pushed to production in a non-user visible way. The goal of a dark launch is to enable testing in a production setting while minimizing risk of causing bugs or outages. Features that have been dark launched are then tested and then slowly made available to end users. Often this is done by exposing the new feature to a small specified group of users, or alternatively to some % of traffic which can be increased over time.
While popularized by companies such as Facebook and Etsy, who have significant traffic patterns that are not feasible to reproduce in development environments, the roots of this technique go back much further. In the early days of the web, developers would often create new pages and new tools on a site with no visible links to those pages; if you new the right url structure you could gain access but there was no way to discover these pages existing. When a new page was completed, links would then be added to the rest of the site, and the feature would be "turned on".