Doing just one type of exercise every day can put excess strain on a particular muscle group, which increases the possibility of injury. That's where cross training comes in. When you cross train, you're basically mixing it up with different kinds of exercises so that you don't overexert certain muscles. So in other words, don't just focus on building muscles; work a cardio routine into your workout as well, or vice versa.
For example, instead of pumping iron for hours on end, you could run on a treadmill for 30 minutes, and then lift some weights to build muscle strength. You could then cycle for another 30 minutes. Cross training can help ensure that you're working out your entire body and not putting too much stress on any one group of muscles.