InfluxDB's Flux is a powerful functional data scripting language used for querying, processing, analyzing, and acting on data, particularly time series data. It offers various features like custom functions, which allow developers to create reusable code snippets that can be applied to specific operations, making the code more efficient, readable, and maintainable. Custom functions in Flux are built using a simple syntax and can be used within scripts or pipelined together with other operators for complex calculations. This enables developers to tailor the language to their application's unique needs and build upon existing libraries and functions.