The marginal product of labour refers to the change in the
total output produced by a firm when it employs and additional unit
of labour.
The firms usually try to maintain the balance between the
number of units produced and the units of labour hired in order to
maintain their profits.
Any profit maximizing firm will employ worker's only until the
marginal product of labour equals the wage rate.
This is because, when firm's hire more workers, the marginal
productivity of each worker gets affected.
If the firm's hire more worker's than the revenues they earn
from producing extra units of output, then they will have to pay
more wages to those additional worker's which may exceed the
revenue they earn from the additional unit's.
This will make them face losses as profits decrease when wage
rate exeeds the revenue earned by them through the workers.