Fibers would be stackful coroutines (every coroutine gets its own stack memory, and local variables inside a coroutine are placed onto that stack). Whereas C++ coroutines are stackless - local data in those gets placed into a magic object that is created somewhere by the compiler.

Stackful vs stackless comes with a few pro's and con's - e.g. it's often argued that stackless requires less memory. However stackful coroutines can allow for preemption at arbitrary points instead of just at yield points.