The GDB Stack buffer displays a call stack, with one line for each of the nested subroutine calls (stack frames) in the debugger session. See Backtraces in The GNU debugger.
On graphical displays, the selected stack frame is indicated by an
arrow in the fringe. On text terminals, or when fringes are disabled,
the selected stack frame is displayed in reverse contrast. To select
a stack frame, move point in its line and type RET
gdb-frames-select), or click mouse-2 on it. Doing so
also updates the Locals buffer
(see Other GDB Buffers).