Those who have been around photography for a while are familiar with 18% gray, but for those who are still learning, this can be an easily overlooked concept. So we're going to try to break it down into some easier to understand concepts (nothing fancy here, folks).
The way your camera's meter works is that it reads reflected light off of the objects in front of it. Since different objects have different reflectance, camera manufacturers decided to use the reflectance of a mid-tone gray (18%) to create a standardized measurement. Now, in your average, evenly lit situation, the light and dark tones will generally even out to that 18% gray reflectance which makes metering those situations easier. Now, if we're talking about an incredibly bright day/light objects or a dark room/dark objects and the entire scene has a brighter/darker than mid-tone gray average to it, your photos will turn out lighter or darker than you intended. The solution to more consistent metering is twofold: Metering modes and practice. If you are shooting a mid-tone subject against a very bright or dark background, switch your metering from Matrix (Nikon) or Evaluative (Canon) which reads the entire scene to Center Weighted/Partial or even Spot metering to get a correct reading on your subject without the light/dark input of the background.
How's that for a photography tip! Feel free to message us or email any questions you might have and your question might end up as a Tuesday Photo Tip!