I don't understand callback [on hold]

I have been trying for days to use callback with nodeJS but I still don't get it. I just want to understand them and how to code it.

Can someone explain to how to do this with callback :

function one() {
    console.log('first function');
    two();
}

function two() {
    console.log('second function');
    three();
}

function three() {
    console.log('last function');
}

one();

via Catheart