A free rider is a person who benefits from something without paying for it.  This is especially relevant in the case of environmental economics.

Free riders take advantage of public goods without having to contribute to them.  For example, if society decides to encourage people to use less of a product that harms the environment and many people actually respond to this call, the environment will improve.  Although the free rider may continue using this product, they will benefit.

This creates a tremendous problem for activists who attempt to start a movement to improve the environment.  Many people, seeing no incentive to join personally in the movement, just continue behaving the way they did before and yet reap the benefits of other people's work.

When everyone acts as a free rider, no benefit at all will come from the proposed action.  Because it is in everyone's personal interest not to participate in the collective movement, the movement is highly vulnerable to failure.

The concept of the free rider closely relates to the Tragedy of the Commons.  Garrett Hardin , the originator of that idea, said that people would overuse resources that were unlimited to them.

Though society as a whole may try to conserve a resource, the free rider can easily take advantage of this by not participating in the conservation effort.

Those who believe in the concept of the free rider and the problems such people create might advocate government policy to require everyone to take part in efforts to improve the environment.  Such people might contend that government action is the only fair and reliable way to prevent environmental problems.