Rent-seeking refers to the behavior of individuals or groups expending resources to achieve public policy decisions that transfer wealth to them at the expense of others. Some examples:
A nonprofit organization might seek for the government to spend taxpayer money on their pet cause, such as protecting the environment or researching a disease.
A workers’ union might want the government to force employers to provide higher wages, more benefits and greater job security.
A corporation might seek subsidization to support an unsustainable business model instead of working to become more profitable.
While the rent-seekers should be faulted for the behavior, it is the government granting rent-seekers what they want that is the real problem. As it shells out more benefits and privileges, government has to collect more taxes to administer and pay for them, thus vastly increasing its size and scope.