IORRA referred to this in his post. In a 'normal' electric circuit the current increases as voltage increases, they are in phase. If you have capacitors in a circuit they have the effect of moving current and voltage out of phase. The illustration I sometimes use in class is to compare a circuit with a capacitor in to sloshing the water backwards and forwards in a bath. The point when all the water is hanging over the tap end represents maximum potential (voltage) but no movement (current), when it's moving past your legs it has maximum movement (current) but no potential (voltage). The result of moving current and voltage out of phase is that power (voltage times current) is reduced, power is 'lost' as reactive load.
Capacitors act to shift current 90 degrees in advance of voltage. This effect is called capacitive reactance.
An opposite effect occurs when you have an alternating current passing through coils of wire, perhaps in a motor. The current creates a changing magnetic field which creates, in turn, a current which flows against the original current, this is called a 'back emf'. The effect of this is to cause the current in a circuit with coils in it, an inductive circuit, to lag 90 degrees behind the voltage. Just as in a capacitive circuit the power is reduced as the voltage and current slip out of phase. This is inductive reactance.
Most circuits have more coils in than capacitors so the two effects are out of balance and power is 'lost'.
The obvious solution is to add capacitors to make the capacitive reactance cancel out the inductive reactance and achieve resonance. In practice this is difficult because the amounts of capacitive and inductive reactance respond differently to small changes in AC frequency, they are constantly changing.