Electro-mines were mines that emitted electric charges across certain distances. If the charge or the electro-mine came into contact with anything, it would explode.
Electro-mines were used as a security defense to guard the Citadel. There were several of them placed on a vertical wall surrounding the landing platform. Around 21 BBY, Jedi Master Even Piell was captured and imprisoned in the Citadel. A rescue team of clones and Jedi encountered these electro-mines when they were scaling the vertical wall to the entrance to the Citadel. They had almost made to the platform but a clone lost his balance and fell down the cliff, getting caught in the electric charge and setting off the mine, killing the clone and alerting the Citadel guards to the Jedi and clones' presence.
The Cartoon Network dataclip for the episode "The Citadel" displays the name as electro mine, without the hyphen. However, the subtitles in the episode itself did hyphenate the name.