Polarized training is where you split your training into really light exercise (aerobic) and really tough exercise (anaerobic) as opposed to doing all of your training at a moderate pace (a mixture of aerobic and anaerobic).
The advantage of polarized training is that you can up your mileage without needing as much rest because so much of your training is light aerobic exercise. When you up your mileage you tend to improve just because you are doing more running (unless you go out and run ridiculously further than your body is used to, it is best to gradually up your weekly mileage to avoid injury).
If you do all of your training at a moderate pace then your body will take longer to recover from training because all of your running at a moderate intensity is more strenuous than most of your running at a low intensity with a small amount of high intensity.
The most commonly recommended ratio of different levels of training intensity for polarized training is: 75% low, 20% high and 5% moderate (race pace). Note that this ratio will change depending on what event you are training for. If you are training for a marathon or half marathon then you would want to do more low intensity training and less high intensity alongside a higher mileage with longer runs, if you were training for a 5k or a 10k then this ratio would probably be right but if you were looking to improve your time for 800m, 1500m or a mile then you would probably want to do a bit more high intensity training however most of your training should still be done at a low intensity to build up your cardio system (on average, 70% of respiration during a 1-mile race is aerobic).