An increment of 1 in exposure value (EV) corresponds to an increase of one F-stop in exposure.
A metered difference in EV between shadows and highlights corresponds to a particular contrast range or lighting ratio. This is important when measuring the contrast of a scene, or when determining lighting ratios, for example, for a portrait. Lighting ratios are especially important when working with flash. Knowing lighting ratios means you have full control over the lighting.
For example, for a given shutter speed, your light meter tells you f/2.8 when you measure the shadows. Then you measure the highlights, and your meter says f/22. That is a 6 stop contrast range, which corresponds to a 64:1 contrast ratio.
Let's say you are shooting a portrait. This could be in the studio with controlled lighting, or in the field. You can easily determine the contrast range between the lighted side and the shadow side of the face. The higher the lighting ratio the more dramatic the portrait. The lower the lighting ratio, the flatter the image. For a standard color portrait, a 4:1 lighting ratio is about right. That means the difference in EV between main and fill light is 2 EV (2 F-stops)
If you use a flash meter, you can easily determine the ratio between ambient light and flash. This allows you full control over the lighting, because you can vary at least the flash intensity, either by changing power output or by changing the flash-to-subject distance.
The chart below shows the difference in metered EV and the resulting lighting ratio:
2:1 some definition through light and shadow, like in a standard corporate headshot
3:1 good tonal separation, like in a better standard portrait
4:1 fairly dramatic lighting, with more impact
8:1 very dramatic lighting
Higher lighting ratios lead to severe loss of either highlight or shadow detail. This may be used for good effect.