A plot device, always unavoidable in a good story, is an element or part of a story (An event) that only serves its purpose to continue the story. A good execution of a plot device will have some continuity and a lasting thing, and is disguised heavily well. A bad one is when the reader/viewer can actually identify it easily.
Here's an example. This is a plot device, but I can't tell if it's a good or bad one, though I hope it's bad.
Dude 1: What's the use of going on? D:
Dude 2: Do it for the world! Do it for your family!
Dude 1: You're right! Let's continue!
Of course, it's not as epic as Legacyblade's slime ordeal, but you get the picture, hopefully...
On the other side, I hope this doesn't turn into another debate (But it shows how intelligent me and Legacy are
)...