what does device node 'none' means in file system?
Usually you mount filesystems which are located on a block device = hard disk, USB stick, ...
Additionally there are some "virtual" file systems such as /proc or /sys which are for interaction with Linux kernel. These do not have any block device associated with them => none is displayed instead.
None is also displayed for tmpfs filesystem. Tmpfs uses RAM for storing temporary files, so there is again no block device associated.