The basics:
Exposure is the amount of light hitting the sensor. Total amount of light is limited by two things. 1. The time you collect light for. 2. the size of the light gathering area (aperture).
In auto mode the camera measures the amount of light in the scene and uses an appropriate algorithm to set the shutter speed (time) and aperture (size of the hole).
To make things confusing the shutter speed is normally reported in nths of a second (ie one hundredth) but represented in camera as just the denominator (ie 100). The aperture is reported as an f-stop which is a function of the focal length and actual cross sectional area of the hole. This gives a consistent measure so that f5.6 is the same effective light gathering power across all focal lengths.
If you double the f.stop then the amount of light gathered halves. If you double the shutter speed (go from 1/200 to 2/200, or 1/100) then the amount of light gathered will double
In practiceThe camera meter will indicate what it thinks is the correct exposure. It has a very mechanical view. As a starting point you adjust one of aperture or shutter speed to get the meter to be at 0. Then decide whether some parts of the scene should be lighter or darker (as a rule the meter will be zero for a 'correct'
- exposure when pointed at green grass or typical caucasian skin, both of which approximate 18% grey).
Then adjust from there. If you think the subject is darker than the scene and want more detail there, add more exposure (longer shutter speed - smaller denominator, or smaller f-number - bigger hole.[**]). Or if it is a bright spot that you want detail in (e.g. the moon [***]) then dial in less exposure.
Go and play. Lots.
Experience[****] will have you looking at the scene, at the meter and making the appropriate adjustments from habit.
Or do as I tend to in changeable situations and use an auto mode with exposure compensation.
ISO is a related matter and is the sensitivity of the light detection. If you have a slow ISO then the camera will need more light for a given exposure[*****], a fast iso requires much less light so shutter speeds can be quicker. Iso numbers work by doubling as well, so ISO 100 requires twice as much light as ISO 200.
- given a constant light source - flash is a different game.
[**] to aid the confusion, a smaller aperture is a bigger f-number.
[***] Actually the moon is sunlit with a constant light so can take standard settings of 1/iso at f16 as a starting point. This is the 'sunny 16' rule.
[****] Most of the good photographers on here still take loads of badly exposed photos, but they add to the learning and experience. You won't get the experience without mistakes that you can criticise and learn from.
[*****] There are trade-offs. The signal to noise ratio drops - not a problem for brightly lit bits but shadows then start to look 'grainy'.