What is this variable?

[vysero]

I am just wondering what this _T("") is doing in the code below:

  CTString   firmwareVer      = _T("");

[jonnin]

the _T is telling it what type of character string it is.
_T appears to be a macro for the L specifier. Why it exists is a total mystery to me.
The L specifier tells the compiler that a string literal is a wide string.

so that is the same as
firmwarever = L"";

[salem c]

https://msdn.microsoft.com/en-us/library/dybsewaf.aspx
It allows you to seamlessly switch between ANSI and UNICODE mode.