These days cars are made with thinner materials than in the past and as result, even a slight impact can cause a damage to the body. Auto body filler is used to fill holes, dents and deep scratches in the body of the vehicle. Very often body filler is referred to as Bondo, which is really a product brand name manufactured by 3M. But since 3M was one of the first companies to produce body filler, it became known as Bondo.
Quality of the product varies depending on the brand. Some are easier to sand, have better adhesion and last longer but might cost a little more. Other cheaper products are harder to apply and can take hours of sanding. But no matter how much we dislike dealing with body fillers, there is no way around them if you are trying to fix auto body.