Cardio (short for cardiovascular exercise) is the term given to a certain type of exercise that keeps your heart rate elevated for a continuous length of time. With strength training, you have a minute of exercise followed by a minute of rest. With cardio, the exercise is non-stop.
Cardio has many benefits including lowering blood pressure, cholesterol, resting heart rate and improving your muscular endurance. Good activities for cardio are those that you enjoy and keep you moving for a long period of time. Some good examples are running, swimming, biking, playing most sports and even house or yard work.
Typically, cardio should be done 3-5 times per week for 30-60 minutes per session. Although cardio is usually thought of as an exercise for those trying to lose weight, if you're trying to maintain or even gain weight, you should incorporate cardio into your routine.
The more cardio you do, the more calories you'll burn and the more weight you'll lose. If you are trying to lose weight, aim for more than just the minimum (3 days per week at 30 minutes per session).
The minimum can be used when you're starting out but aim to increase the amount of cardio you do each week until you are at the upper end of the recommendation (5 days per week at 60 minutes per session). If you aren't trying to lose a lot of weight or are trying to put on muscle, it's fine to minimize the amount of cardio you do.