Enhance project is where you add some more facility or funcationality in the existing system only.
Like suppose right now your system is designed in such a way that it can handle only at max of 1500 stores data. Now your want to add 500 more stores, and same system he wants to implement in the new 500 stores, then for that you need to enhance that system in such a way that it can support all the 2000 stores at atime. Previuos funcationality should not b taken off, arther some more funcationality has to added.
hope this help you in some extent.