Training wheels are an additional wheel or wheels mounted parallel to the rear wheel of a bicycle that assist learners until they have developed a usable sense of balance on the bicycle. Typically they are used in teaching very young children to ride a bike, although versions for adults exist. The term "training wheels" is commonly used in the United States, Canada and Australia to describe the stabilisers' role in learning to ride bicycles.
Attention, these wheels are included in the standard package of 16' bikes. You only need to buy them in case you want training wheels for 20' bike.