std::bind() throwing error when this is not passed

I am trying to understand std::bind(). I am trying to understand the code in the post https://riptutorial.com/cplusplus/example/7541/std--function-used-with-std--bind.

Code is as below.

#include <iostream>
#include <functional>

using namespace std;

class A
{
public:
    std::function<void(int, const std::string&)> m_CbFunc = nullptr;
    void foo()
    {
        if (m_CbFunc)
        {
            m_CbFunc(100, "event fired");
        }
    }

};

class B
{
public:
    B(int x) : y(x)
    {
        auto aFunc = std::bind(&B::eventHandler, this, std::placeholders::_1, std::placeholders::_2);
        anObjA.m_CbFunc = aFunc;
    }
    void eventHandler(int i, const std::string& s)
    {
        std::cout << s << ": " << i << std::endl;
    }

    void DoSomethingOnA()
    {
        anObjA.foo();
    }

    int y;
    A anObjA;
};

int main(int argc, char *argv[])
{
     B anObjB(4);
     anObjB.DoSomethingOnA();
}

I couldn't understand why we are using this in bind call

auto aFunc = std::bind(&B::eventHandler, this, std::placeholders::_1, std::placeholders::_2)

eventHandler is taking 2 parameters and we are binding with placeholders. Not sure why do we need to pass this. If I remove this, I am getting below error.

/usr/include/c++/6/functional:1286:7: error: static assertion failed: Wrong number of arguments for pointer-to-member
       static_assert(_Varargs::value
       ^~~~~~~~~~~~~
main.cpp: In constructor ‘B::B(int)’:
main.cpp:34:27: error: no match for ‘operator=’ (operand types are ‘std::function&)>’ and ‘std::_Bind&)>(std::_Placeholder<1>, std::_Placeholder<2>)>’)
         anObjA.m_CbFunc = aFunc;
                           ^~~~~
In file included from main.cpp:10:0:
/usr/include/c++/6/functional:1929:7: note: candidate: std::function<_Res(_ArgTypes ...)>& std::function<_Res(_ArgTypes ...)>::operator=(const std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {int, const std::basic_string, std::allocator >&}]
       operator=(const function& __x)
       ^~~~~~~~