main.lua is the entry point of your quest. Everything starts from here at runtime. Of course, you can make other script files, possibly organized in subdirectories, and call them from main.lua. For instance, you will probably make a title screen and then start a game.

Here is an example of main script that does almost nothing:

function sol.main.on_started()
  -- This function is called when Solarus starts.
  print("Welcome to my quest.")
end

function sol.main.on_finished()
  -- This function is called when Solarus stops or is reset.
  print("See you!")
end

See the Lua API for more information about Lua scripting in Solarus.