"A power, event, person, or thing that comes in the nick of time to solve a difficulty; providential interposition, esp. in a novel or play."It's not a good thing. It's cheating.
The term comes from the time in performing arts when, if the plot had gotten hopelessly away from the main characters and the playwright had utterly written himself into a corner, suddenly GOD INTERVENES. In the form of a figure lowered into the scene in a basket. Yeah, seriously.
So the hero is suddenly face-to-face with the dragon, and oh whoops, that's right, he lost his sword to that boggy tart in act 2, crap, I didn't think of that. So all he has now is a salad fork... well, shit. Oh! I know! GOD INTERVENES and turns his fork into a bazooka! Ta-dah! That's some good plot writing, huh?
No. It's some crap plot writing, and the reason is that the only satisfactory way for a conflict to be resolved is for THE PROTAGONIST TO INTERVENE. That's what your damn protagonist is for!
Now, in children's books, this is a particular problem. It's especially tempting, because children experience outside powers that providentially interpose themselves all the time-- they're called parents. Parents swoop in to solve kids' problems, give them things they couldn't have gotten by themselves, and save them from danger. That's real life.
But it's not real storytelling. So the next time you're tempted to lower a parent or other powerful adult into your plot to make things easier, DON'T.
To paraphrase G. K. Chesterton,
We don't tell children stories to teach them that there are dragons. Children know there are dragons; they meet them every day. We tell children stories to teach them that dragons can be slain.