What is WINVER?
WINVER determines the minimum platform SDK required to build your application, which in turn will determine at compile time which routines are found by the headers.
You can use this to verify, at compile time, that your application will work on Windows 2000 (0x0500), for example, or on Windows XP (0x0501).
MSDN's page on Modifying WINVER and _WIN32_WINNT goes into this in more detail.
WINVER defines the minimum Windows system the program can run on. There's a more detailed explanation at MSDN. What #define WINVER 0x0501 means is that the program requires Windows XP or Server 2003 to run, and that it therefore can use Windows functionality up through that release.
WINVER means Windows Version. In a nutshell, if you're building for a particular version of Windows, some APIs are available that are not available on previous versions.