The phrase "penny drop" means suddenly understanding something or having a realization. It refers to the idea of a coin dropping in a person's mind, signifying that they have finally understood an important concept or idea. Full definition
Nikki Reed has been wearing her «Starburst everywhere, which is probably because it gives a little edge to any outfit (and might have something to do with the pretty penny she dropped for it, too!) (people.com)
From proceeds from donated vehicles, thrift store and book nook sales, to pennies dropped in dog and cat donation jars at local businesses — every bit moves us closer to a time when all animals are valued and all animals are treated with kindness and compassion. (animalsave.org)
I started making changes to my writing just to get Rob off my back, and then the inevitable penny dropped — he was bloody right. (tiredoldhack.com)