The :lua command is provided by Neovim as a way to execute a Lua chunk.
I can use it to, for instance, execute a print statement.
:lua print('Hello, World!')
I could print out something more interesting like the full path of the current
file using vim.fn.expand with %.
:lua print(vim.fn.expand('%'))
Or as the helpfiles point out, I can see the value of some expression by
including a preceeding =.
:lua =jit.version
See :h :lua for more details.