Fall means it is in the sign opposite its sign of "exaltation". It's as if the planet is more comfortable in one sign, & one house, than all the others because of its nature. If you take an owl, which is a night creature, and place it in the sunlight, it won't like it one bit. (an example) Many people have mostly planets in "detriment" and "fall" and that's not to say it's no good, it just explains its amount of comfort with its sign location, and also defines its power, or light. I'm explaining something I'm learning about myself, so forgive me if I don't clearly explain, but I'm trying. One looks at the whole picture of a chart to get a sense of the reality of the person. An exalted planet isn't necessarily better, it all depends on everything else. It sounds much worse than it is. There are alot of great web sites with information about rulerships of signs. It is more a "traditional" approach to astrology, but one well worth considering and studying. It truly opens doors to help you understand the inner workings of the parts of the whole chart. For example, if Mars is in cancer and Moon is in Aries, the moon and mars are then in mutual reception. They like each other, help each other, etc. But, they could be like two drunks helping each other get home (not very helpful to one another, really) OR something else depending on aspects, placement (angular house?) etc.
hope that helps.