Flashpack is an anti-ship weapon whose design originates (as per WC4 novelization) from a Kilrathi attempt at giving medium fighters anti-capship weaponry.
There's not really any fine details on how it works, other than a comment from Pliers (chief mechanic aboard the BWS Intrepid, your/Blair's base for much of the game) about it being similar to old strip-fusion bombs (which themselves are never explained), but the basic idea is that it burns through the hull (if it's thin or old enough... Vesuvius-class armor was too thick/tough for an FP to work from outside), and ignites the atmosphere of the target ship, cooking everyone alive.
It was produced by Tolwyn's secret project, an organization based on genetically engineered "super pilots", for use in their main strike craft, the Lance. There's no further mention of it, but since the FP is only really useful as a terror weapon against thin-skinned transports or other civilian craft, it's highly unlikely that any remain, except perhaps in the few pockets of Project personel Tolwyn mentions as being stashed away in secret places, near the end of the WC4 novelization. It's certainly not something deployed by Confederation forces, at the very least.