The -g flag causes TiddlyWiki to be installed globally. Without it, TiddlyWiki will only be available in the directory where you installed it.
If you are using Debian or Debian-based Linux and you are receiving a node: command not found error though node.js package is installed, you may need to create a symbolic link between nodejs and node. Consult your distro's manual and whereis to correctly create a link. See github issue 1434
Example Debian v8.0: sudo ln -s /usr/bin/nodejs /usr/bin/node