What is metering?
When the camera measures how much light is falling on the scene, this is called metering. In the old days we used to use a light meter. We carefully picked a mid-grey part of the landscape, or took a reading off whatever we wanted to be correctly exposed (usually someone's face), and then translated the meter's suggested settings to suit our creative priorities.
This is still the most accurate way to get the right exposure.
But 21st century cameras pretty much do away with the need to use light meters, unless you're working in a studio. Now you can rely on the camera's built-in light meter, plus the instant histogram feedback you get.
How does the camera meter?
Just as it's critical that you know what the camera is programmed to do when it comes to fixing exposure (it turns everything mid-grey), so you also need to know what it has been programmed to do when it measures the light in first place (metering). There are 3 kinds of metering that the camera can use, and you can switch between them (check your manual). In each one, the camera is programmed to do something different. Your camera may have a different name for each kind of metering, but the principles are the same:
1. Matrix, or evaluative metering
This is usually the default setting. The camera takes a broad reading from everywhere in the frame. Useful for beginners, but if the background is light (like the sky), and your subject is just in the middle, the light background will skew the exposure and your subject will not be correctly exposed.
2. Centre-weighted metering
The camera gives priority to whatever is in the middle of the frame. This is a better all-purpose setting to use, if you generally want what is in the middle of the frame to be correctly exposed.
3: Spot metering
You pick one very small area of the frame to take a reading from, usually one of the small rectangles that you can see in the viewfinder that the camera uses to show where it has focussed. This is the most accurate way of getting a correct exposure for a single part of the image.
Download chapter 6 from Book 1 here - it has all the information about histograms, metering and exposure compensation. It also has a short quiz so you can test your understanding of histograms:
How to correct exposure mistakes quickly: Exposure Compensation
The camera will either have under exposed or over exposed; those are the only 2 things to fix. You either switch to Manual mode and compensate with more/less aperture, shutter speed or ISO, or you can use the exposure compensation function. This function lets you dial in between 1/3 and 2 or more full stops of compensation (depending on your camera) without having to fiddle about with aperture, shutter speed or ISO settings.
This section is optional, but if you're interested, find out how to use exposure compensation from your camera manual. It is probably a dial on the top of your camera. In the viewfinder, look for a scale that looks a bit like this:
The scale shows you what's happening when you move the dial. The indicator underneath will move left or right as you dial in plus or minus exposure compensation. So if the histogram showed a slightly underexposed image, you could dial in +1 stop of exposure compensation to let more light in, and the camera will increase the aperture by 1 stop (or the shutter speed, or the ISO - it depends on your camera and the mode you are using). Check your manual to see exactly what your camera will change. You have less control over the exact settings than if you are on Manual mode, but this is a very quick and easy way to get the correct exposure without having to look away from the viewfinder.
The Workbooks are available on Amazon
Take a look at the workbooks if you haven't yet. The "Look Inside" feature is enabled on Amazon so you can flick through before you buy. There's also a short video on this page that gives you a guided tour: