D’Alembert’s principle — UPSC IFoS 2014 Physics PYQ

Using the concept of D'Alembert's principle, show that the generalized force can be defined as Q_j = \sum_i \bar{F}_i \cdot \frac{\partial \bar{r}_i}{\partial q_j} , where \bar{r}_i is the Cartesian coordinate of the i^{\text{th}} particle experiencing external force \bar{F}_i and q_j stands for the generalized coordinate. Discuss the significance of the above expression.