Let's say there's a magnetic field B passing through a conductor loop with an area A. The magnetic flux through the loop, if B is constant and perpendicular to the loop, is B multiplied by A. Now let's say the magnetic flux is changing. This can be caused by a combination of the strength of B changing, A changing, or the orientation of B or the loop changing.
Lenz's law states that the changing magnetic flux induces a current in the loop such that the induced current creates its own magnetic field to oppose the change in the flux.