What does mean "разверзлись хляби небесные?"
Well in the Bible it means heavy raining of epic proportions, but this word could, as of now, is known basically due to this particular phrase (and actually is used in the Bible few more times).
Old Russian хлѩбь stands for
стремнина (rapids), generally speaking, some fierce and turbulent flow.
You can find this definition here. Here's a screenshot (it's easier to do rather then retype in modern spelling, also losing some context):
Also it worth to mention that in modern Russian "разверзлись хлюби небесные" could be used, though this usage is way too rare, in a joking context - when talking of bad weather.