It means that if the aeroplane is disturbed in roll, and the pilot does nothing about it, the aircraft's bank angle will continue to increase (rather than tending to roll towards wings level).
Generally regarded as a bad thing in training and IMC-use manually flown aeroplanes; only a mild nuisance in a VMC only aeroplane, and probably not noticed in something normally flown through an A/P. The degree of instability is also relevant - most commonly you'd measure it as time to double bank angle from a small (e.g. 15°) disturbance - if it takes 20 seconds you'd probably regard it as effectively neutral and not worry much, if it takes 3 seconds in a training aeroplane I'd regard that as unsat(HD) verging on unacceptable depending upon the precise aircraft role.
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