my TL asked me to go through the below code while going through the code i stuck at at what is busboy.on('field', function() } is can any one solve it and why we are using reqpipe method at the end ?

   app.post('/fileupload', function (req, res) {
    var busboy =  Busboy({ headers: req.headers });
    //  const tmpdir = os.tmpdir();
    const uploads = {};
    const fields = {};

    busboy.on('file', async function(fieldname, file, filename, encoding, mimetype) {
      //  console.log(`Processed file ${file}`);
      uploads[fieldname] = filename;
      // console.log(uploads.file.filename);
    var saveTo = path.join(__dirname, uploads.file.filename);
    const writeStream = fs.createWriteStream(saveTo);
    file.pipe(writeStream);
   
    });

    busboy.on('field', function(fieldname, val, fieldnameTruncated, valTruncated, encoding, 
    mimetype) {
  
    fields[fieldname] = val;
    console.log(fields)
    });


    busboy.on('finish', function() {
    res.writeHead(200, { 'Connection': 'close' });
    res.end("That's all folks!");
    
    });
  
    return req.pipe(busboy);    
    });