Luminosity is an intrinsic(natural) property of a star. It is the amount of light that is emitted from the star on its surface. This means that regardless of the observer, the luminosity of the star is constant.$^7$
Once the light is emitted from the star, it travels radially outward. If you assume that there are spherical shells around the star, each of these shells receive the same amount of energy, the energy released by the star. However, the shells further away from the star have a larger surface area. This means that per unit area, there is less energy.$^7$
The surface area of these shells change a square factor. This is apparent in the bellow diagram.
Apparent brightness, also called flux, is how bright the star appears to be to an observer at a given distance.$^7$
$$ F = \frac{L}{4\pi d^2} $$
Where $F$ is the flux, $L$ is the luminosity, $d$ is the distance from the light source.$^7$
To find the units:
$$ F = \frac{W}{m^2} $$
Hence, flux is watts per meter squared.$^7$
The solar constant is the apparent brightness of the Sun from Earth. This varies across the orbit.$^7$
The apparent magnitude system is another way of measuring stellar brightness introduced by the ancient Greeks. The dimmer the star, the higher magnitude it is assigned.$^6$
In general, for every 5 magnitudes of difference in brightness, the objects differ by a factor of 100(in flux).$^6$
Absolute magnitude is the apparent magnitude 10pc away from the stellar object.$^6$