So, you want to be a cinematographer? Look cool with a big camera like this guy on the right? (This guy is Roger Deakins. If you know his body of work, he needs no introduction. If you don’t, check it out.) Perhaps you’re playing it modest and would like to just shoot better videos on whatever camera you have, be it a smartphone, a DSLR or and old Super 8 camera?
Whatever is your goal, you need to understand what Roger is doing on the second picture. What is that strange little box in his hand? And what is he looking at?
It is a light meter. And he is taking a reading to see how much light there is so that he could either decide on the aperture he’ll be using or the amount of light that he still needs to add (or take away) from his scene.
Metering light. This is what cinematographers do. Or at least used to do.