Minetest has a scripting API (Application Programming Interface), which is used to program mods (short for "modifications") for the game, extending its features and adding new items. This API is accessed using an easy-to-use programming language called Lua. More specifically, the version of Lua is 5.1.
The only thing you will need is basic programming knowledge. See Lua for details.
Lua API Reference
Use this file to look up functions, core features of the Lua API, etc. This file is maintained by the core developers of Minetest and every change to it goes through a quality control process.
This document explains everything you need to know about the Lua API in concise form, but may be daunting for newbies to read. Therefore, it's recommended that you start with a tutorial.
The Minetest Modding Book is a friendly introduction to Minetest modding and game creation, introducing you to various aspects of the API.