Its a delivery which passes at or about the head height of batsman without bouncing from the pitch. Such delivery is considered as NO Ball and repeated beamers from a bowler can cause the bowler to be banned for the remaining of the match.
A ball that does not bounce (usually accidently) and passes the batsman at or about head height. If aimed straight at the batsman by a fast bowler, this is a very dangerous delivery (and generally frowned on)
In the terminology of the game of cricket, a beamer (less commonly beam ball) is a type of delivery in which the ball, without bouncing, passes above the batsman's waist height. Such a ball is dangerously close to the batsman's head, due to the lack of control a bowler has over high full tosses. This type of delivery can result in injuries to the batsman, and the penalty for this type of delivery is an immediate no-ball signal given by the umpire.