A tongue port, also referred to as a "juba port" or "tongue loller", is essentially a rubber tongue depressor that can be attached to a pony's bit. It is designed to keep the pony's tongue from getting over the bit by keeping it pressed down. The great thing about these is they are extremely cheap (< $2 from Horseloverz), can be purchased at most tack stores, fit nearly every bit, and can be cut smaller with scissors if it is too long and/or causes your pony to gag.
A rubber bit port (above left), and a rubber bit port (above right) attached to a rubber bit. The rubber bit is the most common type of horse bit used to make the bit gags (via addition of a leather strap) found in many fetish stores. With a bit gag strapped tightly in place, the attached bit port may cause gagging due to how far back the tongue port will extend into the mouth. In this case, you can either cut the tongue port so it is not as long, gradually train the pony to control its gag reflex (since they are pretty cheap you can buy a couple and have a few with different varying lengths), or tell your pony to deal with it :).
Below is a simple diagram illustrating how the rubber bit port attaches to a horse bit (I also have a step by step photo tutorial illustrating how to attach a tongue port to a bit). It is a very simple and cheap device that you can easily attach to pretty much any bit gag or horse bit. In fact, the bit port will usually be less than the cost of shipping, so you might consider purchasing a few of them or dropping by a local bio-horse tack store.
As you can see from the diagram above, the bit port has a curved portion which fits against the mouthpiece of the bit. Then, while holding the bit in this curved portion, you pull the tongue depressor portion through the top hole. That's it. The bit port is now attached to the bit, and will be held securely in place. It might take a couple tries to get the tongue depressor portion through the top opening due to the tightness of the fit, but otherwise the procedure is relatively simple.
With the rubber bit port attached to his bit gag, your pony will drool uncontrollably. This can be either a good or bad thing depending on your personal tastes. The drooling caused by the restricted tongue movement with a tongue port is worse than with any other gag I have used, so you may wish to give your pony water at periodic intervals when wearing this for a longer period of time. Personally, I'm ambivalent about the drooling: one the one hand it's humiliating and makes me feel all the more helpless (good thing), but on the other hand, it's really annoying and gross to have drool all over me (bad thing).
Another great aspect of the tongue port is its complete effetiveness in preventing any intelligible speech. Though it does not muffle the volume of sounds your pony can make, it does make it impossible for your pony to make anything even close to intelligible speech. I do really like the feeling of restricted tongue movement as well, and having to fight to control my gag reflex when the reins are pulled really adds to the feeling to being totally controlled (and makes for a very responsive, obedient pony).
There are some bio-horse bits that have a port built in, and in those cases, it may not be necessary (or appropriate) to attach a rubber bit port. If you have a bridle that allows you to interchange bits, these are useful alternatives to a separate tongue port.