"The term ‘expose to the right’ refers to the histogram associated with an image. Typically, for a shot to be well exposed, we are taught to aim for an even spread of tones across the histogram, peaking in the middle, and tapering off at the edges. When ‘exposing to the right’, the idea is to push the peak of the histogram as far to the right hand side as possible, i.e. overexpose the image, without clipping any highlights. The resulting file, when processed back to the correct exposure, will contain more tonal information and less noise in the shadow areas, maximizing your image quality."
"The diagram below tries to illustrate the distribution of tones for each stop of the dynamic range of the sensor. The top image shows the seven different stops capturing different portions the dynamic range from the darkest through to the brightest tones, however the bottom diagram shows those stops but sizes them relative to the number of tonal levels that each stop captures. As you can see, number of tonal levels captured by the brighter stops is significant compared to the stops at the lower end of the dynamic range."