C++ unresolved externals

Does anyone know what this means?

1>  Generating Code...
1>BlankWindowDXbaseImpl.obj : error LNK2019: unresolved external symbol "public: __thiscall DXBase::DXBase(void)" (??0DXBase@@QAE@XZ) referenced in function "public: __thiscall BlankWindowDXBaseImpl::BlankWindowDXBaseImpl(void)" (??0BlankWindowDXBaseImpl@@QAE@XZ)
1>BlankWindowDXbaseImpl.obj : error LNK2019: unresolved external symbol "public: virtual __thiscall DXBase::~DXBase(void)" (??1DXBase@@UAE@XZ) referenced in function "public: virtual __thiscall BlankWindowDXBaseImpl::~BlankWindowDXBaseImpl(void)" (??1BlankWindowDXBaseImpl@@UAE@XZ)
1>main.obj : error LNK2019: unresolved external symbol "public: void __thiscall DXBase::shutdown(void)" (?shutdown@DXBase@@QAEXXZ) referenced in function _wWinMain@16

1>main.obj : error LNK2019: unresolved external symbol "public: bool __thiscall DXBase::initalize(struct HINSTANCE__ *,struct HWND__ *)" (?initalize@DXBase@@QAE_NPAUHINSTANCE__@@PAUHWND__@@@Z) referenced in function _wWinMain@16
1>C:\backup\development\directXworkspace\BlankWindow\Debug\BlankWindow.exe : fatal error LNK1120: 4 unresolved externals

Answer

It means that you have unresolved external symbols.

What are symbols? Symbols can be anything from variables, classes, member functions or functions.

Why they are unresolved? Some part of your code (or libraries that you are using) rely on these symbols and they are not being found because you are not linking the correct library or implementing them.