Faraday's law states that an induced emf is proportional to?

Faraday's second law of electro magnetic induction
The magnitude of emf induced in a closed circuit is directly proportional to the rate of change of magnetic flux linked with the circuit

The magnitude of an EMF produced in a conductor is proportional to the rate of change of magnetic flux-linkage experienced by the conductor.

This refers to the size of induced voltage that can be produced be moving a conductor through a magnetic field and means.....
 .... a greater EMF( induced voltage) is produced when the conductor:

.. moves faster through the magnetic field (that's the 'rate of change' bit .... ie 'per second')
.. moves through a stronger magnetic field (measured as magnetic field strength B)
.. sweeps through a bigger area of magnetic field (area A)

 mathematically this law can be expressed E = ( B x A x n) / t ...

 ... (B x A x n) is called the 'magnetic flux linkage' the conductor has with the field (if the conductor is a coil of wire, A is the internal area of this coil and n the number of coils - to give the effective total area)

.... t  . gives the rate ('speed') of change of all this flux stuff and is often included in  the equation when the area is expressed in m²/s ( as A/t ..  the area 'sweep-rate'.)