Correct exposure of an image means that the image isn't too bright, or too dark. If too much light is hitting the sensor, your highlights (the brightest parts of your image) will "blow out" or "clip" and appear bright white. By contrast if too little light is hitting the sensor your shadows (the darkest parts of your image) will be "crushed" and appear pitch black.
When filming or taking a photo, your camera sensor is collecting light and converting that light information into data. If your image is lets in either too much or too little light, the sensor cannot record that data, resulting in an over or under exposed image respectively.
There are 3 in-camera variables you can change to get correct exposure, the shutter speed, aputure, and ISO.
The shutter speed, measured as a fraction, is the duration of time the sensor is exposed to light. A low shutter speed, such as 1/50th, will let in more light than a fast shutter speed, such as 1/200th.
The aperture, measured in f-stops, is the size of the the iris in the lens. A low aputure, such as F1.8, will let in more light as it is larger than high apertures, such as F16.
The ISO, measured numerically, represents the sensor's sensitivity to light. A low ISO, such as 100, will detect less light than an high ISO, such as 6400.
You can also get correct exposure by changing your environment. You can add more lights to a scene to increase the exposure, or take lights away.
Using these variables together allows us as filmmakers to get correct exposure every time.
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