std::stack is a standard container like std::vector or std::list. It is a "stack", so a last-in-first-out (LIFO) container.
For offline use if you want to compile it, you need to #include <stack> header (and #include <string> too).
The Kata tests already have these headers included so I gracefully left those includes. (after years thinking about this, maybe it was a little confusing choice...)