Static member within a class template

I was coding a class template to implement a Singleton when an issue occured.

While having a static pointer in a .h file, it wouldnt compile because of a linker issue (lnk 2001 and lnk 1120 on vs 15).

I Simplified my code to have the more understandable issue :

#pragma once
#include "SingletonTemplate.h"
int main()
{
    SingletonTemplate<int>::test();
}

my class SingletonTemplate.h (there is no .cpp btw)

template<class T>class SingletonTemplate
{
public:
    static int myInt; 
    static void test()
    {
        SingletonTemplate<T>::myInt = 1;
    }
}

I read on several posts that the further declaration of the static variable can solve this issue. I added this outside of my function (but still in the .h) :

template< typename T >
int SingletonTemplate<T>::myInt;

Doing that solved my issue but I have no clue why. So, if someone could explain me what is the purpose of this line, it would be awesome.

Thanks in advance