Alright, there have been a zillion posts on this, but I'll sum up. The cycle refers to the various forms of nitrogen in our system. It starts out at ammonia, which is a by-product of decomposition, and is also present in animal waste (poop and urine), so it is an unavoidable problem. Ammonia is highly lethal to marine animals, even 1 part per million (ppm) can kill them. However, aerobic (using oxygen) bacteria grow that consume ammonia. They turn this ammo into the next form of nitrogen: nitrite. Nitrite is also highly lethal, as little as 10 ppm can kill. But, over time, aerobic bacteria grow in the tank that consume nitrites. These then produce the next form of nitrogen: nitrates. These are less lethal, but still problematic at high levels. Generally, over 100 ppm can start causing deaths. There are bacteria that develop that consume nitrates and turn them into nitrogen gas, which bubbles harmlessly out of your tank. However, these bacteria are hardest to cultivate because they are anaerobic (require an oxygen-free environment). This means they will only grow at the bottom of a deep sand bed, or in the middle of LR. Frequently, the tank will never be able to grow as much of these anaerobic bacteria as is needed to take care of all the nitrates in your system, so water changes are needed reguarly to export nitrates from your tank. So, cycling your tank means adding something that produces ammonia (best choice is a single raw shrimp from the grocery store which will decay and produce ammonia a little while after it is dropped in your tank). Bacteria will then grow to turn it into nitrite. Then bacteria grow to turn these into nitrate. At this point, cycling is effectively finished. Eventually, you'll want to have bacteria established that will consume your nitrates, but this isn't necessary before you start stocking your tank with live animals. Regular water changes are necessary to keep those trates down. Cycling can take as little as 2 weeks (some would argue less) or over a month. Basically, it is however long it takes your tank to grow the bacteria needed to turn the ammo to nitrites, and then the bacteria to turn the trites into trates. You have finished cycling when your ammo and nitrite are at 0 ppm, and your trates should be under 20 ppm. If you don't cycle first, then you are adding your animals into a system full of deadly poison. Is it posible they will survive long enough for the bacteria to grow and consume the poison? Yes, but why put the animal through hell? HTH
best of luck, feel free to come back and ask more questions.