NWSync is a custom content loader for persistent worlds that runs Neverwinter Nights Enhanced Edition server. It allows players who want to play on a server that relies on custom content (content that is not part of the official distribution of the game) to download the content directly while connecting to the server from the game.
Before NWSync, custom content had to be distributed manually, requiring players to download .hak and custom .tlk files and putting them into the game folder.
Beside simplifying the process of downloading the content, NWSync also solves the problem of redundant content that player used to download when playing on different servers that however partly use the same custom content files. If one of the servers already requires a file X, this file will not be downloaded when attempting to connect to a different server that uses the same file.
More information as well as (slightly outdated) documentation can be found on GitHub.
Some additional (out of date) information is available in this technical document (Google Docs version).
Potential issues and solutions
1) You have to provide a valid URL for the NWSync for your server. Bear in mind that as for 20.09.2020 the master server included a syntactic URL validation for the NWSync URL. If the validation would fail, the server would (as a technical side effect) not be listed on the list of known servers in the multiplayer section of the game. For instance, a link nwsync.server.com would fail validation. http://nwsync.serever.com would pass.
2) You see errors with files over 15MB. Solution: Lower those file sizes - this limit is there for console compatibility. Note: As of client patch 8193.35 this limit has been raised to 64mb.