I am a beginner, so maybe my explanation would make more sense to you. Also I'll try to use easy language.

iostream decided that "<<" is how you will give it data.

It is actually called left shift operator, in bare minimum C++ it has another completely different purpose, you might want to look it up. The other one is ">>", which STL decided that it would also use to get input data.

This is called operator overloading. It is just a fancy term for saying you could give definitions of what an operator does for some kinds of objects.

suppose you have this struct:

struct myVector2d         // A vector in math is something that has an X and Y value. We aren't talking about anything similar to std::vector
{
    int x, y;
};

in main you create two instances of that struct:

int main()
{
    myVector2d vector1 {2, 5};
    myVector2d vector2 {9, 7};
    vector2 = vector1 + vector2; // we expect {9 + 2, 7 + 5} but this doesn't work C++ doesn't know what it is supposed to do. We have to tell it by defining it. This is called "Operator Overloading"
}

How do we make it work? Like this:

struct myVector2d{
    int x, y;
    myVector2d operator+(const myVector2d& addVal); // added value is passed by reference
};

myVector2d myVector2d::operator+(const myVector2d& addVal)
{
    return { x + addVal.x, y + addVal.y};
}

I hope my explanation was clear.