What does podManagementPolicy: Parallel change in a StatefulSet?

Answer: Scaling starts and stops pods together instead of in ordinal order

Explanation

The default OrderedReady policy starts pods in order (0, 1, 2), waits for each to be Running and Ready, and removes them in reverse order. Parallel drops that ordering for scaling. It doesn't change rolling updates, which still replace pods one at a time in reverse ordinal order.