The Bestboy mark-it black positioning marker is a tool used by the bestboy (the gaffer's assistant) to mark positions for actors on the ground or set. It is typically used in film and television productions to help actors know where to stand or move to when they are performing.
The marker is black so that it is easy to see against a variety of light backgrounds, such as white walls or floors. It is also made of a durable material that can withstand being stepped on or walked over.
To use the marker, the bestboy will simply mark the spot where the actor needs to stand or move to with a small X. The actor can then use this as a reference point to ensure that they are in the correct position when they are performing.