Lua is used for writing rules and plugins in Rspamd. Rspamd exposes various core functionality through its Lua API.
Lua plugins are a powerful tool for creating complex filters that can access practically all features of Rspamd. Lua plugins can be used for writing custom rules which could interact with Rspamd in many ways such as using maps and making DNS requests. Rspamd is shipped with a number of Lua plugins that could be used as examples while writing your own plugins.
Please read the guide about writing Lua rules.
You can select the module you are interested in in the list on the left. Some of the modules are implied, for example, if you see task in the list of your function arguments it is assumed that you check for rspamd_task module. There is also some super globals available, for example rspamd_config.
Please read the following document that describes the main principles of writing async code in Rspamd.