A micro-explosive was a minuscule explosive device. Micro-explosives were sometimes used by assassins, as the explosive could be implanted inside of a target without their knowledge and detonated remotely.
Mobara was assassinated by Silaran Prin via the use of a micro-explosive implanted behind his right ear. Constable Odo hypothesized that the bomb had been injected via a remote-controlled hunter probe while he slept. (DS9: "The Darkness and the Light")