Most device only contain a single hardware stack. Based upon the number of ISR supported, there can be a varied number of 'Shadow Registers' which the stack is 'reserved' and 'restored' when a cooperative task is interrupted and must be handled by a preemptive one.
Base on the screen shot take, the code execution is at that stack depth when the program was halted. So the 'firmware' is not using the final stack points at this point in operation. There could be other points that it does.
The stack will Pop and Push based upon the functions being entered during code execution. Each function call will 'pop' onto the stack with the return address meant to inform the device when it's 'pushes' that task out of the stack, it will return execution from the address given. This is the traceback from where it was last called from.
If code execution goes over the stack size allowed in device, it will overflow; and the return address can be come corrupt and the program cannot find the 'bread crumbs' back home.
Typically if warning are being presented, the compiler is attempting to say there is a risk. It's not always 100% accurate, and you can usually refer to the generated .map file to get more detailed information regarding the call trees, occurring in the program. This can be helpful in finding code modules which carry a specific depth of execution to include into operation.
I hope this answered the question, and has been helpful.