what is flux and what are field lines?

Answer

You must first ask yourself what a field is. You can define a field if, for example, a force can be defined at any point in space (as is the case with the gravitational force or the electrostatic force). Imagine such a vector field as a bunch of arrows (vectors) which, at every point in space, have the direction and meaning of the force that would act on that point, and have a magnitude proportional to it.

Once you have that vector field, you can define field lines. These are lines that are tangent to the field vector at every point (similar to streamlines in a flowing liquid)

The flux of the field lines through a surface is proportional to the number of field lines that pierce that surface. This therefore not only depends on the size of the surface, but also on the orientation of that surface relative to the field lines.