In major championships, a large screen rests diagonally across the table preventing each player from being able to see his partner and one of his opponents. A small slit in the screen allows a tray to slide from one side of the table to the other.
The players bid, using bidding boxes, by placing the bids on this tray and repeatedly sliding it from one side of the table to the other. During the play, a flap comes up to allow all players to see the dummy and the cards the others play.
One purpose of screens is to prevent players conveying information to their partners through their mannerisms or eye contact. Another benefit is that if there is a delay in the return of the bidding tray, it will often be unclear whether your partner or an opponent has been thinking.