The term “free rider” refers to a person who gets the benefits of certain actions without bearing any of the costs of those actions. This is a concept that can be used in economics, in political sciences, and in other areas.
In economics, an example of a free rider is a person who does not join a labor union in a workplace that has a union. That person will usually get the benefits brought by the union. They will get the wages that and many of the protections from arbitrary actions on the part of management that the union has negotiated. They will get this even though they have not contributed any money to the union.
In political science, the free rider problem is a problem for interest groups. Let us say you believe strongly in gun rights but you do not want to pay dues to the NRA. The laws the NRA gets passed (or that it prevents) benefit you. You get the benefits without paying the price.
The implication of this is that it can be hard to make people pay the price for the things that they benefit from.