NodeBB is licensed under the GNU General Public License v3 (GPL-3) and GPL says that you need to distribute the code or changes that use any GPL code under same license.
Does this mean I need to opensource my website when I integrate NodeBB github version ?
IANAL, but as I understand it, if you don't make changes to NodeBB core code (code from github.com/NodeBB/NodeBB), then you have nothing to worry about. If you do make changes, then you must release those changes under the GPLv3.
Use the plugin / theme system to make your changes and you can use whatever (or no) license you want with it. That's why we did this in the first place so that you have absolute freedom to do whatever you want