You can use Lua script to configure Neovim, but why might you want to do that? The Vim and Neovim API are written in C, and Lua runs in the C engine; since Lua is significantly easier to write and read than C or even Vimscript, Lua can be used to write complex configuration directives.
Another more explicit reason to use it is because many useful Neovim plugins are written in Lua (like the autocompletion plugins used when setting up Neovim as an IDE) and require extensive configuration directives that can clutter up your init.vim file if they’re embedded but can be neatly separated into external scripts called by Lua with a single line, which makes it much, much easier to maintain a lot of plugins.
While you can write your entire Neovim configuration file in Lua, this document will assume that you have an existing init.vim file and wish to use some Lua within it. There are two ways to call Lua from your config file: embedding the Lua code directly, and calling an external script.
Embedded Lua
If you want to keep all you configuration in a single big file, you can embed the Lua portion between a block like so:
lua << EOF
lua print('hi from lua')
EOF
You can see a big config example using this in the nvim-cmp example setup.
Calling an external Lua script
If you want to break your configuration into multiple files, you can use the require function and call the script from a location within Neovim’s runtimepath. The standard location is within a lua file within the nvim folder where your init.vim lives:
.config
|-- nvim
|-- autoload/
|-- init.vim
|-- lua/
|-- myluamodule.lua
|-- othermodules/
|-- anothermodule.lua
|-- plugin/
|-- syntax/
Then the following will load myluamodule.lua from init.vim:
require('myluamodule')
If the external script is located in a subdirectory it will be called like so:
require('othermodules/anothermodule')
-- or
require('othermodules.anothermodule')
Note that require() not only searching through all lua/ directories under the runtimepath, it will also cache the module on first use. Calling require() a second time will not execute the script again and instead return the cached file. To rerun the file, you can remove it from the cache manually first:
package.loaded['myluamodule'] = nil
require('myluamodule') -- read and execute the module again from disk
To run other Lua scripts automatically on startup, they can go in the plugin directory in the runtimepath. No require function is used in this case.