Product stewardship is the inclusion of waste disposal measures in the distribution chain of an industrial product. That is, paying for the safe and proper disposal when you pay for the product, and relying on those who sold it to you, to dispose of it.
Those who advocate it are concerned with the later phases of product lifecycle and the comprehensive outcome of the whole production process. It is considered a pre-requisite to a strict service economy interpretation of (fictional, national, legal) "commodity" and "product" relationships.
The most familiar example is the container deposit charged for a deposit bottle. One pays a fee to buy the bottle, separately from the fee to buy what it contains. If one returns the bottle, the fee is returned, and the supplier must return the bottle for re-use or recycling. If not, one has paid the fee, and presumably this can pay for landfill or litter control measures that dispose of say a broken bottle. Also, since the same fee can be collected by anyone finding and returning the bottle, it is common for people to collect these and return them as a means of surviving. This is quite common for instance among homeless people in U.S. cities.
However, the principle is applied very broadly beyond bottles to paint and automobile parts such as tires. When purchasing paint or tires in many places, one simultaneously pays for the disposal of the toxic waste they become. In some countries, such as Germany, law requires attention to the comprehensive outcome of the whole extraction, production, distribution, use and waste of a product, and holds those profiting from these legally responsible for any outcome along the way. This is also the trend in the UK and EU generally. In the United States, there have been many class action suits that are effectively product stewardship liability - holding companies responsible for things the product does, which it was never advertised to do.
Rather than let liability for these problems be taken up by the public sector or be haphazardly assigned one issue at a time to companies via lawsuits, many accounting reform efforts focus on achieving full cost accounting. This is the financial reflection of the comprehensive outcome - noting the gains and losses to all parties involved, not just those investing or purchasing. Such moves have made moral purchasing more attractive, as it avoids liability and future lawsuits.