The 5 Elements are Chi, Sui, Ka, Fu, and Ku. They are ways of moving or being in combat situations, every technique in the martial arts can be related to at least one of these elements. However one does not want to get stuck on one particular element, you should be able to flow from one to another to another.
Chi the element of earth, it is applied to combat by transforming yourself into earth. What this means is that you are solid as rock, unmoveable as a mountain. When you are like earth you are grounded or rooted and you let your opponent run into your technique. This does not mean that you just let him hit you, you step to one side and let him hit your punch, or perhaps a forearm, or many thousands of different possibilities.
Sui the element of water, is applied to combat by being fluid like water. Like a wave you go back and then crash into your opponent with your technique. You take your oppent in one direction, usually the one that he is already traveling in, then you rapidly change his direction usually resulting in a throw.
Ka the element of fire, is applied in combat by being as intense as fire. When you are fire you attack your opponent almost relentlessly, being offensive and never letting the pressure of your attack off so that he has no time to counter attack, all he can do is block or be hit. The key to this strategy is to attack from many different angles, because attacking from only one or two angles is easy to block or defend against.
Fu the element of wind, is applied in combat by being quick like a tornado. The goal of this element is to blow through and around your opponent preferably ending up behind him before he has time to react. From there it is easy to take him down.
Ku the element of void or no-thing, is used in combat by not being. I think this is the hardest element, both to describe and do. To use void you want to empty your mind and just let your training do the work. You are not thinking about what you are going to do or what your opponent will do, you just react. You cannot do this without lots of training, your techniques need to be as effortless mentally as walking. I think that its a lot like riding a bike, once you learn how to ride one you don't need to think about it anymore, you just do it and can think about other things.