I looked at Get containing path of lua file, and I can see that:
print(debug.getinfo(1).source)
... results with e.g.:
@/Users/e/test.lua
Apparently, as a path, this is a string - but what is the "at sign"/"at character" @ there for; what does it mean?
It was otherwise quite difficult finding anything useful related to this, by doing an internet search for Lua "at sign" (even with quotation marks)
It means that the function was defined in a file.
From the Lua 5.4 Reference Manual: 4.7 The Debug Interface
The fields of lua_Debug have the following meaning:
source: the source of the chunk that created the function. If source starts with a '@', it means that the function was defined in a file where the file name follows the '@'. If source starts with a '=', the remainder of its contents describes the source in a user-dependent manner. Otherwise, the function was defined in a string where source is that string.
The Lua manual lists other uses of @. For example warn("@off") can be used to turn off the emission of warnings.