In a nutshell, this plugin allows you to enable automatic reordering by post date for specific categories' subcategories.

For example, if Category A has three subcategories, B, C, and D, and automatic re-ordering was enabled for A, then any post in B, C, or D will change their ordering when seen via A, by bumping that category to the top.

So when originally the order was B, C, D, a new topic or reply in D will trigger re-ordering, and the new order would be D, B, C.

Install via the admin control panel, and restart NodeBB. In the plugin settings, enable automatic re-ordering for whichever categories you like. The ordering itself applies only to the subcategories of the enabled category.