That usually means it is for a newer version of android than your device has, or is in some way incompatible. Or the file is corrupt. Least that's my experience usually from going from ICS to jellybean on my tablet.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer? | Terms of Service