It is a variable angle attachment that can be used with a number of Gossen hand meters.
From the Gossen meter attachment brochure:
View attachment 393274
FWIW, it is usually a lot better to include a still photo in a post that asks questions like this - I tried to screen grab one from your YouTube video, but you didn't hold it still for long enough to make it easy!
In my experience, those attachments are most useful for metering stationary subjects where picking a small portion of the subject to take a reading off is important for accuracy, but a spot meter is not available.