Do I need a flash exposure meter if im using a Nikon D80 digital camera?

This lets you set the function of the very flexible built-in flash.

Hint: Be sure to hit OK after changing anything, otherwise the D80 ignores what you think you changed!

TTL (default) lets the flash work normally and gives great exposures all by itself.

M (manual) lets you force the flash to one brightness value. It adjusts in full stops from full power down to seven stops below, shown as 1/128 power.

I use this if I'm using the built-in flash to trigger my studio strobes or if I'm trying to set up something tricky which isn't working in the usually perfect TTL mode.

Repeating Flash is a gimmick. It lets the flash fire manually many times in fast succession. You might play with this in a dark room with a long exposure to make a stroboscopic image of something in motion.