ISO, aperture, and shutter speed are the three sides of the exposure triangle. All three effect the brightness of the image and all three change one other aspect of the photo or video you are shooting.
Shutter Speed
The first part of the exposure triangle is shutter speed. Shutter speed changes how long the sensor sees the scene you are exposing. It makes the image lighter and darker, as well as the motion blur, depending on how fast it is. Faster shutter speeds give you a darker image and less motion blur. Slower shutter speeds give you a brighter image and more motion blur.
This is different from your frame rate as the frame rate decides how many times your sensor will look at the scene. In video, you want your shutter speed to be double your frame rate to get the proper motion blur. This is the first part I am mentioning because it’s set to a specific value when you shoot video.
Like I said, it should be twice your frame rate. This means for 24p, you should set the shutter speed to 1/48 (or 1/50 because most cameras don’t have the option for 1/48). For 30p, the shutter speed should be 1/60. And for 60p, set the shutter speed to 1/120.
Aperture
Aperture changes the amount of light getting to your sensor. The aperture is marked on lenses as ƒ-stop or t-stop. The higher the number goes, usually up to 22 to 32, the smaller the aperture opening in lens gets. The lower the number gets, usually down to 1.2 to 2.8 (but can get lower on some lenses) the bigger the opening gets.
With the opening larger, more light gets into the lens to the sensor, but it comes with a trade-off. The smaller the opening, the longer the depth of field, meaning more of the shot is in focus. The larger the opening, the shallower the depth of field, meaning little is in focus. When the aperture is wide open on some lenses, just breathing can pull the subject out of focus.
This is second because it will allow you to choose your depth of field instead of settling for something shorter or deeper than you want.
ISO
ISO changes the sensitivity of your sensor, essentially brightening or darkening the image. The higher you set the ISO, the brighter the image will be. As a side effect, the image also gets noisier.
Noise, or grain, is the very small colorful dots in dark areas of the image. It’s what the sensor processes when it cannot read the details on that part of the image. The higher you set the ISO, the more it creeps into brighter areas. If your ISO gets high enough, you can end up with the whole image getting covered in noise.
This one is last because you can always add or subtract light to give you the ISO you are going for.
For more information on the exposure triangle and other camera functions, check out these cheat sheets.