Defold uses a programming language called Lua. It’s very commonly used in games, for scripting of plugins, ui or all game logic.
If you have no prior experience of programming in any language then you need to start from the absolute beginning. My recommendation is to read the book Programming in Lua: Programming in Lua (first edition)
If you know a bit of programming then maybe this is enough: https://learnxinyminutes.com/docs/lua/
Finally make sure to also read this: Lua programming in Defold