What is mean of "asynchronously" in "set_clock_groups"?

It's when:

a) The clocks can't be analyzed synchronously because they are truly asynchronous, so there's no valid setup/hold window since the clock relationships are constantly changing. There are circuits that can be build between asynchronous clocks where the user wants some timing relationship, such as in hand-shaking circuites.

b) Most importantly, it's when you've designed the system so that timing between the clock domains doesn't matter. At this point you're telling TimeQuest not to analyze these paths and the fitter not to optimize them.