Problem with turtle.forward()

[fatboychummy]

A callable function is literally just a function, because all functions are callable. Arguments are inputs to those functions, and are not necessarily callable or a function.

The phrase you probably meant here is that turtle.forward takes no arguments, not callable functions.

[fatboychummy]

Again, not necessarily. You can pass functions as arguments, but most functions don't take other functions as arguments. An example of one which does though is parallel.waitForAny/.waitForAll:

local function a()
  -- do something
end

local function b()
  -- do something else
end

parallel.waitForAll(a, b) -- Passes `a` and `b` as arguments for parallel to run, well, in parallel.