what is function (err, results) { in the below?

I am starting with nodejs and read basics on callback, here i have a code,

 exports.handler = (event, context, callback) => {
    var bx = require('barcode-js');

    // Set default values.  
    var params = {
        input: './1.bmp',
        type: 'qrcode'
    };

    bx.analyze(params, function (err, results) {
    if (err) {
       callback('There was an error processing this image: ' + err)
    }
    results.forEach(function(result) {
        callback(null,'Result type :' + result.type + '\t'+'Value: '+result.value);
     });
});
};

what is happening bx.analyze(params, function (err, results) { in this line. why can't we just use bx.analyze(params) ?

via face turn