...translated into English literally as "piece of resistance", referring to the best part or feature of something (as in a meal), a showpiece, or highlight. It can be thought of as the portion of a creation which defies (i.e. "resists") orthodox or common conventions and practices, thereby making the whole of the creation unique and special. The phrase gives the sense that the referred-to element is the defining essence of the whole, that part that makes it memorable or gives it its unique character.
There would be fresh perch, asparagus, and _la piece de resistance_— first-rate, but quite plain, roast beef, and wines to suit: so much for the eating and drinking.
Leo Tolstoy -- Anna Karenina
Then they brought in the piece de resistance, a carcass roasted whole and carried by four men.
Robert A. Heinlein -- Glory Road
"But the piece de resistance," he continued, tapping his chest, "is right here.
Sarah Dessen -- Along for the Ride
Kitty and Levin would be of the party, and that this might not be obtrusively evident, there would be a girl cousin too, and young Shtcherbatsky, and la piece de resistance among the guests—Sergey Koznishev and Alexey Alexandrovitch.