This is the monopsony diagram with the union added. The employer would like to hold employment at L*, where the marginal cost of labor equals what a worker is worth, and pay only Wm, the wage the supply curve says will attract that many workers. The union would like the wage at Wu, the height of the demand curve at the same L*, which is the most the employer could pay for that many workers.
So both sides push employment to the same place, below what a competitive market would hire, and the two of them are left bargaining over a wage anywhere between Wm and Wu. Economics does not say where in that range they land: that depends on the relative bargaining power of the two sides. The slider moves the demand for labor, which widens or narrows the range.