It sounds like you’re asking about (specifically version 1.5.2 ) and what useful features it offers.
Eaglercraft was a web-based port of Minecraft that allowed users to play the game directly in a web browser (like Chrome or Firefox) without needing to download the game client or own a premium Minecraft account. It achieved this by compiling the Minecraft Java source code into Javascript (WebAssembly) using the TeaVM compiler. 1.5.2 eaglercraft
Running an Eaglercraft 1.5.2 server was different from running a standard server. It sounds like you’re asking about (specifically version 1