1. Using a Light Meter during Manual Mode (Also known as an Exposure Meter)

2. Why use Manual Mode instead of Auto Mode?
Why you would want to go to all this trouble when you can put your camera in Automatic mode and it will just take care of everything? The answer lies in the fact that you, not your camera, know exactly what kind of photo you want to take. Wide depth of field? Blurry Background? Captured/frozen movement or blurred? Manual Mode lets you decide.

3. The light meter is an internal function of the camera that gives you a visual indication of how dark or light the image is according to the camera. The light meter is this little thing you see in Live View or through the viewfinder.

4. The light meter is a readout that shows whether the 3 values below are going to result in a photo that is properly exposed – that is, a photo that is neither too light or too dark.
The small triangle hovering over the zero in the light meter shows that the exposure is correct.

8. UNDEREXPOSED

9. HIGH ISO IS CAUSING A LOT OF NOISE
Light meter says photo has good exposure

10. Same, perfect exposure but less noise/graininess and different settings!

11. Metering Modes
The camera evaluates pixels from the scene. Depending on your metering mode camera setting, it may check the area of the whole frame or just a fraction of it.
There are two basic metering modes that tell the camera how much of the frame to check.
Digital cameras offer more, but once you know the principle, you can decide which one you want to use:
Center Weighted Mode
Partial/Spot Metering Mode

12. Matrix/Center weighted mode
Whatever this is called in different cameras, it is indicated as a blank rectangle.
This mode tells the camera to compare all pixels in the frame to the middle gray and calculate the average brightness. If the average brightness is less than the middle gray the cursor goes to the left, otherwise, to the right.
This mode will give you an overall impression if the scene is well exposed. The problem is it won't tell you if there are way overexposed or way underexposed spots in the frame. That is the reason I do not use that mode when I see very bright or very dark spots in the scene.
                                                                    
Matrix/Center weighted mode

13. Partial/Spot Metering Mode
This tells the camera to look at a small area around your current focal point.
If you place your currently selected focus point to the part of the scene you want to evaluate, the light meter will indicate if it's lighter or darker than the middle gray color. This is the metering mode I usually use because it's more precise.
                                                               
     
Spot metering mode