what does grayed-out packets mean?

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It just means that they've matched a coloring rule that uses that pale gray color. Scroll down in the Frame section and the very last two items will be the coloring rule name and the coloring rule syntax. These packets seem to have matched the very last coloring rule of Wireshark's default coloring rule set. It for for Ethernet broadcasts that did not match any other coloring rule higher in the list.