The clipboards are usually whiteboards with a diagram of the basketball court. The coach can use whiteboard markers to draw different plays, strategies, formations, and movements on the diagram to guide players to where they need to be on a given play. This is commonly used during timeouts, where players gather around the coach while he sets up a play.
Clipboards are beneficial for players as they can quickly learn a play at a quick glance. Instead of needing to put the players on the court to understand a given play, they all can visualize it during a quick time out on the sidelines. It's easy to see the court as a whole when looking at the board. This is much easier for coaches as they can show players what they want them to do instead of describing it.