what ooes this error mean?

error LNK2019: unresolved external symbol _WinMain@16 referenced in function ___tmainCRTStartup

and how to resolve it?

Perhaps you forgot to define the global main function.

You need to make sure you're compiling a "console" program (which expects main()), or a GUI program (which expects WinMain())

IIRC, goto project options -> c/c++ -> linker -> system -> find option subsystem, set to console.
This is taken from memory because I don't have the IDE to check right now. But it's there somewhere.
Or you can create a new project and copy your files over. Be sure to select a win32 console project.