This reference manual is 103 pages long. The reference manual is the official definition of the Lua language.
For a complete introduction to Lua programming, see the book Programming in Lua by Roberto Ierusalimschy.

Lua is a powerful, fast, lightweight, embeddable scripting language. Lua combines simple procedural syntax with powerful data description constructs based on associative arrays and extensible semantics. Lua is dynamically-typed, runs by interpreting bytecode for a register-based virtual machine, and has automatic memory management with incremental garbage collection, making it ideal for configuration, scripting, and rapid prototyping.