"Karari" and the derived "kara-kara" are used in multiple ways in this song. "Kara-kara" is typically a repetitive or continuous "karari" (noise made by hard objects coming in contact), but when used with something that isn't sound it typically describes complete and strong clearness, and "karari" can carry the connotation that it is pleasant (although not necessarily). Given the abstract and hence flexibly usable meaning, it can translate into a lot of things. (The song only uses some of them, by the way. And I risked oversimplification with this explanation...)
 "Flowers" are cherry blossoms. The recurring (in poetry) fluttering is either when they are caressed by the wind when they're still on the tree, or, much more often, as they fall from the trees. In this line it's the former. But later in the "traverse into the stage of fluttering flowers" stanza it's the latter. Cherry blossoms bloom once a year en masse, last only one week and fall en masse. They carry a lot of symbolic significance in the Japanese culture and literature. See, for example, Mono no Aware.
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