MCEF/JCEF for Minecraft is essentially the Chromium browser engine embedded into the game through a mod. It acts as a core backend that lets Minecraft render real web content, HTML/CSS/JavaScript interfaces, and browser-based views inside the client instead of relying only on vanilla-style GUI systems. In practical terms, it is the foundation another mod can use to display things like custom web-driven menus, browser screens, online panels, video-capable interfaces, or advanced app-like UI inside Minecraft. It is not mainly a content mod by itself; it is more of a technical framework or dependency that gives other mods Chromium-powered browser functionality in-game.
All credit goes to the original authors for MCEF at https://www.curseforge.com/minecraft/mc-mods/mcef. This mod ports over their mod to the newer version of Minecraft 1.21.5. Still requires original connection to https://mcef-download.cinemamod.com to download the files.