Not only does dry mouth make it hard for you to speak, or eat, but even when drinking, the mouth seems to have reconstituted itself. The water being guzzled down to slake your thirst feels like it is hitting an impenetrable skin barrier. But perhaps what is most important about the dry mouth as a symbol is where a dry mouth comes from, and why it appears in dreams.
If you have a dream in which you have a dry mouth it means that you want to learn something. Usually it means you have a thirst for knowledge and if you can’t satiate that thirst you need to at least try by going out and learning something. You need to take on a few new responsibilities and teach yourself something. Now in your life would be the best time to do that, to let yourself learn something new. Even if it is not something totally new, going out and learning something would be a good reason for you to go out and do all kinds of things. Take this dream as a reason for you to go out and have new experiences.
Having a dry mouth in your dreams can also mean that you are bordering between waking and sleeping. Your mouth is dry in real life because you feel to sleep with it open or because it is really hot. You are noticing this in your sleep because of how acutely aware your mind is of your surroundings and of your physical body, even while your conscious mind is not tuned in. This is a special ability that does not come along to just everyone, so feel grateful for it.