I think it means that it is still usable but it is on its way already.

Struts have oil and gas in them the gas is there to prevent oil form bubbling when going over lot's of bumps. Gas under pressure is what makes the shock expand when it's new. If the shock still has resistance but does not come up by itself it means that it is still operational but there is not much gas left in there. If you are working it hard (going fast on a long bumpy road) it will loose it's damping abilities quicker than a shock that still has a lots of gas inside.