Minecraft Wiki
Not to be confused with Mojang.
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This feature is exclusive to Bedrock Edition. 

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. It is used in Bedrock Edition's Add-on system with its purpose being enabling low-level systems like animation to support flexible data-driven behavior for both internal and external creators, while staying highly performant. [1]

Mojang provides official documentation which goes into details of how the language works and where it is used.