Reactive Power means power that is flowing in a system but not comes in account for billing,actually if we want to realize how reactive power flows ,take voltage and current equations(in terms of sinusoidal values,with phase shift between V and I) and multiply,and solve ,we get Two components ,one with power frequency ,and other with twice the power frequency, so the first one is active power ,second one is reactive power

But Reactive power is must for Power flow through a transmission line, Running of Motors (to create Flux), and many applications ,But a minimum reactive power only necessary for a transmission line for charging line distributed capacitance,Loads connected drawing reactive power through this line only ,so Transmission line capacity (of sending active power) degrading ,so we should decrease amount of reactive power and improve amount of active power that to be sent through a transmission line,For this Purpose we place compensators at loads to compensate reactive power requirements of loads