A dungeon is much like a cave in that they usually have monsters and scary things and dangerous things inside them, but it is a step up from just a cave because it has a little more organization to the environment and you get the idea that in some sense the dungeon was built, or constructed to meet a certain set of specifications, which may appear differently in dreams.
If you have a dream about a dungeon and want to know what it means then the first thing is to find out what was going on in the dream of the dungeon. Did you dream that you were living in the dungeon? That means that you feel like a hermit and that in real life you should think about being a little more introverted. If you have a dream in which you are venturing into a dungeon then this type of dream means that you need to be more adventurous in your daily life and you will be rewarded for it. You may not know exactly where life will take you, but sometimes not having every little detail of your life mapped out for you is a good thing. Sometimes this will serve you well.
A dungeon is a scary place and it can be tough and complicated to traverse it, but in the end there is usually a steep reward for having done so. Perhaps your dream means that whatever trials you are going through now, however hard your life gets, you will be rewarded for all of your hard work in the end.