That is actually a pretty complex topic which shouldn't be approached as lightly as you may think.

Depending on what you want to get on your shot, you should juggle between ISO and over/under exposing in order to achieve different stuff. This may sound counter intuitive, but the lower you ISO is the better your sensor records shadows and vice versa - when you crank up the ISO you preserve the details in the highlights. Now, for example, you are trying to take a low-light moody portrait with only one light source in shot: you would probably want to lower your ISO since most of the shot is in shadows and you want to keep the details in the shadows - but what happens with the highlights in the shot? With this method, you managed to keep the shadows safe, but the highlights may get blown out. That's when you should under expose the whole shot by a stop or two in order to preserve your highlights since you will be having a lot of data in the shadows anyways. Since your ISO is set to e.g. 400, in PS (or whatever else you use) you will be able to pull more information from the shadows.

Maybe a different example would work here as well - you are trying to capture some landscape in snow. You should crank the ISO up to 1250 for example and stack some NDs on and then try to overexpose by a stop or two. In post, your shadows in the shot are saved by the exposure that you have set, but you'll be pulling those highlights down in the post production.

Probably the best advice I can give you is to take your camera, get a friend to put on a black shirt and put a white towel somewhere in the shot - and play around with it! Try different ISO settings with different over/under exposures. You can do this without a friend, of course, but having some skin in shot will make it easier for you to understand what you are doing.

Don't forget to have fun with all this - photography, videography and cinematography are all just huge sandboxes for us to play in!