What is the Lenz's law?
Lenz's law states that,
If an induced current flows, its direction is always such that it will oppose the change which produced it.
Lenz's law is in accordance with the law of conservation of momentum.
The illustrate it's importance, let us look at a simple example,
If we move the N of a bar magnet towards a closed coil, there shall be a induced current in the coil due to EM induction. If the induced current flows such that the electromagnet so generated has it's south pole towards the N of the bar magnet, the bar magnet shall be attracted towards the coil with an ever increasing acceleration.
In such a case, we could design a perpetual machine which could continue to generate electrical energy without expending much mechanical energy. This is in violation of the law of conservation of energy.
Thus, the current in accordance to Lenz's law flows such that the North pole of the electromagnet faces the N of the bar.
As such we have to apply a force to take the bar magnet further towards the coil.
The induced current is thus opposing the motion of the magnet which has produced it.
We are expending mechanical energy and the energy appears as electrical energy in the coil.
Thus according to Lenz's law, the current so produced shall always tend to oppose whatever has produced it so as to be in accordance with the law of conservation of energy.