I have seen this device that is used widely in filmmaking. What is this device and what is it used for?
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That is an incident light meter.
It's used in both motion and still photography, although with modern cameras the built-in light meter is usually so good (and so convenient) that separate meters are not as essential as they once were.
There is a difference, though, because a camera's built-in meter measures light reflected back to the camera, whereas this measures light falling on the subject. (Or more specifically, on the little white dome, which helps mimic the way light falls on a real three-dimensional subject.) A key difference is that the reflective meter can't tell the difference between a dark subject in really bright light and light subject in dim light — you have to use exposure compensation when the subject isn't middle brightness. An incident meter gives an absolute value.
These devices often have special features for measuring bursts of light from flashes — useful for photography and not so much for cinema. Some fancy models can also measure color temperature for white balance. This model looks like the Sekonic L-358, or a close cousin.
More on this here: Why use a dedicated light meter instead of the one built into the camera?