I don't understand what is the meaning of "flux" .I just know it has some relation between eletromagnetism.Please explain what is flux to me.
It could mean field lines (e.g. magnetic flux) per unit area or flow (photons) per unit area. Or generally flux is the rate of transfer of fluid, particles, or energy across a given surface.
magnetic flux is the magnetic field measured in webers. You should read up on electromagnetic induction. http://en.wikipedia.org/wiki/Electromagnetic_induction
A flux is the value of the surface integral of any vector field. If the field is constant over the area of interest, the flux is simply the product of the field and the area. Conversely, the magnitude of the field is given by the flux per unit area.
Provided the area and field are perpendicular, of course (the surface integral being the integral of the dot product of the field and the area over the surface). AM