Your fish will excrete ammonia, which is toxic. In the ocean, there are bacteria that feed on ammonia and convert it into nitrites, which are also toxic. Fortunately, there are also bacteria that feed on nitrites and convert them into nitrates, which are much less toxic. Algae and some anaerobic bacteria feed on the nitrates and convert them into nitrogen gas, which is harmless.
Cycling your tank is basically just growing a population of these bacteria that convert ammonia to nitrites to nitrates (and with a DSB, nitrates to nitrogen). The way I've cycled my tanks is to add the sand, live rock and water, then toss in some food (about the amount I'd feed to fish in the tank), and continue doing that on a daily basis. The ammonia will rise, then start to drop as nitrites start to rise. Those will eventually drop as nitrates start to rise. Then you can do a water change or two to get your nitrates into an acceptable range (maybe around 10 ppm), and you're ready to start adding livestock.
