As mirageii suggests, lots can be said about metering.
But some easy thumb-rules are:
- If you want to control Depth Of Field, user aperture priority. The metering will adjust everything else to obtain equivalent exposure.
- If you want to control to what extent moving objects are frozen or blurred, use shutter priority mode. The metering will adjust everything else to ...etc.
- If you aren't sure about either DOF or are in the mood to control the degree of blur/freeze of moving subject, you can go to fully automatic mode and the camera will chose for you.
Different cameras have different levels of sophistication in terms of what they can meter for.
- For sure the light level (to adjust either shutter or aperture accordingly - or both in "full auto")
- Also the temperature (which helps auto white-balance)
- And of course the auto-focus, dynamic, static and a wealth of other variations.
- Then there are variations of "expanded dynamic range" where the metering will dial back the exposure based on the highlights to get more shadow-detail.
- And various "scene recognition" type reference databases in-camera where it not only meters, but makes adjustments according to the type of image it believes/recognizes that you're taking.
Metering has really taken off, sophistication and precision-wise, with digital photography. That's what makes it so darn easy to take properly exposed shots.
Anyone with an ancient history with film will remember the outrageously high number of "complete misses" when everything was fully manual..lol.