The "Call Stack" window is can be used to trace your script down in its execution. Each time a function is called, it adds an item to the top of the stack. When this function returns, it removes the item the item at the top of stack which is the call to that function. Double-clicking on the stack will bring back the state of Lua's environment at the moment of the call. That is, the values of the locals and globals at the moment of the call will be set temporarely until you resume the script. The wellow arrow is used to display the current call location of the script. When a different location has been requested by double-clicking on an item, a green triangle will be used to display the currentely requested call location.