A function of the Nikon Creative Lighting System, Flash Value (FV) lock maintains the same flash exposure for your main subject when you're shooting a sequence of photos, allowing you to zoom in on your subject, change the composition or adjust the aperture, all without altering the intended exposure.
Once FV Lock is set on your Nikon Speedlight, the flash value remains at the locked in setting until the FV Lock is pressed a second time, the camera's light meter times out or the camera is turned off.