There are many types of bit mouth pieces, made of many different materials. All are designed to enhance the rider’s aids and give clear signals to the horse, while being comfortable and easy for the horse to carry. Many bits have a port. It may be very shallow, or it can be very high, with other additions that spin or rattle. Basically, a port is an inverted "U" in the middle of the mouthpiece on some bits.
A very low port may appear to be only a slight rise, while a very high port can be quite high—over two inches in very high ones. A very low port allows room for the horse's tongue and is sometimes called a tongue relief. Some snaffle bits have a port, but they are most commonly seen on curb bits. Some ports will also have a roller that connects the two branches of the “U”.
A high port, in addition to giving the tongue relief, also places pressure on the palate of the horse's mouth when the reins are pulled. Used improperly, a high ported bit can be very painful or damaging for a horse's mouth. When you hold the bit in your hand, that bit will appear to stick straight up, but when it sits properly in the horse's mouth, the port lays down flat on the tongue until the rein aids are used. Then, depending on how the reins are handled, the port may rotate up slightly. It shouldn’t rotate enough to press hard into the palate of the horse’s mouth.
Ports that are solid, rather than made of a “U” shaped bar are actually called spoons or spades, although they look somewhat similar. These do not provide tongue relief. The spade or spoon may include elaborate rollers or "crickets", double mouth pieces and are only used on very well trained horses by skillful riders who understand the severity of the bit they are using.
Generally, bits with ports do not have jointed mouthpieces, although there are some that have joints on either side of the port. These are attached with a pin-like hinge, rather than a link.
Ports are found on western, English and driving bridles. Generally, they are to be used in more advanced riding, where the horse must be more responsive to the rider's aids. You'll see ports on many western bits used in rodeo events and reining, and often on the curb bit of a dressage bridle. If bits with ports are misused, they can be very severe compared to the same bit without a port. Without a curb chain to keep the bit from rotating in the horse's mouth, bits with ports can be used in a way that is very painful for the horse. Generally, for pleasure riding, you won't need a bit with a port, unless your horse benefits from the space it gives its tongue.