It's wire (usually copper I believe) that is wrapped around the explosive charge of some grenades... the wire has little notches all over it, so when the grenade explodes the wire breaks at each notch, so you get lots and lots and lots of little tiny copper fragments for shrapnel.
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Shot four puppet governors in a line,
Shook all tha world bankers, who think they can rhyme,
Shot the landlords, who knew it was mine,
Yes, its a war from the depth of time!