Exposure Triangle

The "exposure triangle" is the name for the group of three primary settings which control how much light is let into the camera - i.e. the amount of light "exposure" within your photo or video. The three settings that make up the exposure triangle are the ISO, shutter speed, and aperture.

ISO is the camera's sensitivity to light - a lower ISO (ex: 100) is going to allow less light in than a higher ISO (ex: 6400). The tradeoff however is the higher ISO you have, the more grain or "noise" your image will contain, thus degrading the details and quality of it.

Shutter speed is how quickly the shutter of the camera closes. Higher shutter speed lets in less light but creates less motion blur, whereas lower shutter speed allows in more light but creates more motion blur.

Aperture is the amount of the light which the lens lets in. Lower aperture (larger) allows in more light and a higher aperture (smaller) allows in less light. Additionally, a larger apeture creates a more shallow depth of field and a smaller aperture creates a wider depth of field.

The key to creating a properly exposed image is balancing these three setting. Typically the aperture will be kept as low a possible in order to create a greater separation of the subject from the background (higher amounts of bokeh), the shutter speed will be approximately double the frame rate (if I'm shooting at 30 fps shutter speed will be 1/60), and ISO will be kept as low as possible in order to preserve image quality. 

All of these setting are dependent upon lighting conditions though - for example if I'm inside I may need to set my ISO to 2000 in order to see my subject properly, but if I'm outside where it's extremely bright then I'll likely have my ISO as low as it goes (100). If the image still remains too bright I would likely increase shutter speed, or employ the usage of a UV or ND filter.