Excuse me, what does ev mean in photography? What does it have to do with aperture shutter? Is everything a constant combination?

Ev is exposure value, which is a numerical value that represents the light passing ability of a photographic lens by the combination of shutter speed value and aperture value. This is a quantity reflecting the degree of exposure. Initially defined as: the sensitivity is ISO 100, the aperture coefficient is F 1, and when the exposure time is 1 s, the exposure is defined as 0, the exposure is reduced by one step (the shutter time is halved or the aperture is reduced by one step), and the EV value is increased by 1.