It means there are uncaught memory errors which produced a bad share, which got caught by SRBminer and was not submitted. Some of my microns exhibit memory errors but almost never produce a bad share (but a memory error reported is an error caught and "fixed", the mem errors which don't get caught are the problem), but my Hynix ones never display a mem error in monitoring software yet they produce bad shares once in a while. If bad shares by a specific card is above 0.5% (compared to what it produces on its own), you may reduce mem freq by 10mhz or up the voltage. Just 10mhz may work wonders.