A video mixer is a device or piece of broadcasting equipment that allows the incorporation of multiple cameras and/or multiple video signals (e.g. pre-recorded commercials/spots; PowerPoint slides; etc.)
Users manually switch between the multiple devices to produce a more complex broadcast.
Mixers come in a variety of formats including hardware (Roland, Panasonic, Newtek), software (vMix, Livestream/Vimeo, OBS), or both.