A Facebook Watch Party is a feature offered to users to watch either one or many videos together at the same time. So when a business page or a personal profile has gone Live, you as the user will see a button underneath the video saying ‘Start Watch Party’.
As a personal user, this allows you to ‘host’ this video and invite either a group that you’re a part of or friends to watch it in real-time with you and create a private space to message and chat with one another while you watch the video together. You can also create a watch party by creating a playlist of your favorite videos on Facebook, either video that you have uploaded or videos from pages that you follow and then you can all watch them together.
As a business page on Facebook, you can make use of this feature by creating a playlist of videos that you have uploaded and grouping them according to topics, for example, How To, Tutorials or Virtual Tours, etc, and then invite clients and followers to watch them with you. A Watch Party is just another way Facebook is trying to connect friends and users via video.