A plot device is something in a story, be it a character, object, event, goal, or general truth, that advances or maintains the plot.
Most plot devices are inherently neither good nor bad, but some are easier to pull off than others. Plot devices that are great when executed properly include Chekhov's Gun, red herrings, universal translators, and important objects that are the goal of a quest. Plot devices that make us cringe include trivialised rape, Random Orcs, Twu Wuv, and the classic deus ex machina.