Minecraft's Molang parser & evaluator implementation in Java.
Molang is not implemented in Minecraft: Java Edition. Common usages of Molang have been replaced with pure Java code and simpler JSON data files. Third-party libraries are required to load files created for Minecraft: Bedrock Edition in Java Edition, for modders to be able to utilize the formats.
This is one of those libraries.
To quote bedrock.dev...
Molang is a simple expression-based language designed for fast,
data-driven calculation of values at run-time, and with a direct connection to in-game values and systems.
Its focus is to enable low-level systems like animation
to support flexible data-driven behavior for both internal and external creators, while staying highly performant.