A correct exposure is where your images have nice rich color, they are not too light or too dark and you see visible detail in both the highlight and the shadow areas.
When shooting in Auto Mode your camera determines the correct exposure, as accurately as possible, using its reflected-light meter. Three basic elements then work together to create the exposure: aperture, shutter speed and ISO.
If you take a photo of a beautiful landscape but you’re disappointed because the image is washed out and the sky looks white instead of blue you will have what’s considered a less than perfect exposure.
Another example is in how exposure is describe relative to time, such as a night shot of the Aurora Borealis, or Northern Lights. A photographer might say they took the photo using a “longer exposure,” which may have been five minutes duration for the time the camera was recording the scene.