what is req.on('data'( in nodejs code example

Example: res object anatomy nodejs

res = {
    app            :    function(req, res) {},
    chunkedEncoding:    Boolean,
    connection     :     Socket,
    finished       :    Boolean,
    output         :    [],
    outputEncodings:    [],
    req            :    IncomingMessage,
    sendDate       :    Boolean,
    shouldkeepAlive    : Boolean,
    socket         :     Socket,
    useChunkedEncdoingByDefault    :    Boolean,
    viewCallbacks  :    [],
    writable       :     Boolean
}