For the uninitiated, Eaglercraft is a legendary browser-based port of Minecraft. It runs entirely in your web browser using JavaScript and WebAssembly. No download, no Java install, no "sorry, you’re on a school Chromebook."
The term most likely refers to:
Users often search for "eaglercraft 120 1" as a shorthand (dropping the periods). It’s a typo-friendly way of finding version 1.20.1. This version is desirable because it represents the "modern" Minecraft experience without the more controversial changes of 1.21. eaglercraft 120 1