Lenz's law is a rule to predict the orientation and sign of an induced electric field. It states:
In other words, if a magnetic field is passing through a loop and somehow changing, the induced electric field will produces an electric current that flows in whatever direction is necessary to make a magnetic field directly opposing the original one. The actual magnitude of the induced field is usually tiny; Lenz's law only tells where it goes, not how much of an effect it has.
In a way, this can be interpreted as inertia. A conducting loop wants to maintain a constant flux in time. If any change occurs, the loop will make an attempt to cancel that change out. It's a sort of Newton's first law for magnetic flux.