Quoting from Wikipedia: "The Law of Rent states that the rent of a land site is equal to the economic advantage obtained by using the site in its most productive use, relative to the advantage obtained by using marginal (i.e., the best rent-free) land for the same purpose, given the same inputs of labor and capital."
Where "land" is taken as capital, equipment, and/or alternate business opportunities, the employee's wage-bargaining leverage, in the absence of collective bargaining, is what s/he could make by going elsewhere and starting up a new firm. Where no new business opportunities exist, wage bargaining falls to subsistence levels (employers will pay employees the bare necessities for staying alive).
In the economy at large, the situation still remains true. An employer need pay an employee no more than that employee could claim at another job (or by going into business for him or herself), given the employee's skillset.
Given that skills tend to wed to experience, should the employee transition to a different line of work (at which they are less skilled), unless there is a peculiarly high demand for that work, their wages will fall. Also, the employer's surplus (that is, productivity above wages) is governed by the Law of Rent.
Given collective bargaining, through the threat of withholding labor (with skills that, collectively, the employer would be hard-pressed to replace), a negotiation for total compensation (wages, hours, benefits) in which more of the employee's surplus is distributed to the employee may be arranged.
