A commons good is a type of resource that is shared by a group of people, which can be accessed and used by anyone but is susceptible to overuse and depletion. These goods are non-excludable, meaning it is difficult to prevent individuals from accessing them, but they are rivalrous, meaning one person's use can diminish the availability for others. This creates a challenge in managing these resources sustainably, as individuals may act in their self-interest, leading to the 'tragedy of the commons.'