The call stack window gives a list of frames corresponding to the current execution stack for the current thread/task.
The bottom frame corresponds to the outermost frame where the thread is currently stopped. This frame corresponds to the first function executed by the current thread (e.g main if the main thread is in C). You can click on any frame to switch to the caller's context, this will update the display in the source window. See also the up and down buttons in the tool bar to go up and down one frame in the call stack.
The contextual menu (right mouse button) allows you to choose which information you want to display in the call stack window (via check buttons):
By default, only the subprogram name is displayed.
You can hide the call stack window by closing it, as for other windows, and
show it again using the menu Data->Call Stack
.