In electrostatics, area is a vector. It is called as area vector. The direction is always perpendicular to the surface and faces radially outwards.
Symbol($\phi$)
It is defined as the number of magnetic field lines crossing per unit area.
As the angle of magnetic field lines and the direction of the area vector increases, the magnetic flux decreases. In the above diagram, the theta is 0. When theta is 90, the magnetic flux is minimal. Magnetic flux is a function of cos.