It is a double-sided board used by referees to display which player is entering the game ("IN") and which is leaving ("OUT").
Key features of this type of board include:
Double-sided display: Allows visibility from both sides of the field.
Color-coded numbers: The numbers are typically in bright colors like orange and yellow-green to be easily seen from a distance. The orange numbers often indicate the player exiting the game, while the yellow numbers indicate the player entering.
Number range: The board can display numbers from 00 to 99.
Durable design: They are often designed to be drop and weather-resistant.