Sales are generally made to 'early adopters' in the market place. The product gets known more widely. There are few competitors, and profit margins tend to be good. Here, sales are maximised. If the product has mass market appeal, this is when the bulk of potential customers will buy.
Competition increases, and profit margins tend to fall.
Shown by the dotted line on the chart. The product starts to 'show its age'. New and better competitive products emerge in the market. Sales fall and profits dip sharply. Inevitably, however, the life of the product is at an end.