Like flux brightness is energy per unit time per unit area (e.g. ergs per second per square centimeter). How bright something appears to you depends on how much energy (light) it is giving off per second, and how spread out it is over your viewing area. A certain amount of light energy will appear much brighter if concentrated into a small region of emission than when spread out over a large emission region. A tiny lightbulb can seem very bright, even when its total light is small. The apparent brightness of a star is called the apparent magnitude and that is what is measured by a telescope: how much energy does the star put into the telescope's collecting area per second.