At the end of its life, a biodegradable product is likely to decompose without negative effects on the environment and within a short period of time in regard to human time if it finds itself in favourable conditions to break down naturally with the help of microorganisms.
In the best cases, the products biodegrade without leaving toxins in the soil. This is the case for those that are made of plants: they break down into carbon dioxide, water, and natural minerals and can therefore mix into the soil without damaging it with toxic residue. We prefer these over products requiring years to decompose.