The concept of module in Lua took a while to settle down. Modules can be written in Lua or in C, and both kinds can be loaded by using the require function. The syntax is

x = require "foo"

which returns a table x containing what the module foo exports. The require function first checks to see whether foo is already loaded. If not it uses the system variables LUA_PATH (for Lua modules) and LUA_CPATH (for C modules) to search for foo. These variables contain a list of wild-carded paths with a ? for where the foo should appear. For C modules the paths must be in Unix format. RiscLua has in its Boot file

set LUA_PATH rlua:lib.?,?
set LUA_CPATH rlua:solib.?/so,?/so

The riscos module gives Lua access to RISC OS itself.