What is the function declaration called ? ?

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C++

What is the function declaration  called ?? 

I am using a tutorial where "delay " function in implemented in main .
It is unknown syntax to me and I do not understand
what is the advantage of this instead of using "global" function.

Of course my program does not compile as coded.

Any reference "what is it called ? " will be welcome. I'll be happy to ask Mrs Google if I have a clue what it is I am looking at.

<pre> // Our function to compute
    std::function<void(int&)> spin = [](int &iteration) {
        const int work = 1000 * 1000 * 40;
        volatile int v = 0;
        for (int j = 0; j < work; ++j)
            ++v;

        qDebug() << "iteration" << iteration << "in thread" << QThread::currentThreadId();
    };

What I have tried:

Nothing, need definition first to do search.