In more detail
At ISO 100, EV = log2(N² / t), where N is the f-number and t the shutter time in seconds. Raising the EV by 1 halves the light, one stop. Equivalent exposures trade aperture for shutter speed: opening up one stop and halving the shutter time keeps the EV the same. At another ISO the same scene needs EV + log2(ISO / 100).
Why it matters
EV tells you how bright a scene is independent of the camera settings, and it underlies light meters and the "Sunny 16" rule of thumb. Knowing the EV of the scene lets you choose among equivalent settings for depth of field or motion blur. The Exposure Value Calculator converts between them.
Example
A sunny day is about EV 15 at ISO 100: f/16 at 1/125 s gives log2(256 × 125) = 14.97. The equivalent f/8 at 1/500 s, f/5.6 at 1/1000 s and f/4 at 1/2000 s all give the same exposure.