If you are writing in third person, either limited character POV or omniscient narrator, you want the narrator to intrude on the story as little as possible. The narrator isn't a character, and the narrator is not part of the plot. Ideally, you don't want the reader to notice the narrator.
There are times, however, where you let a narrator butt in. Narrator intrusion is often used to set scenes, to provide backstory, and to explain character behaviour or plot intricacies.
Setting a scene
Even when writing in limited, you might have to describe the setting. Best is to let the POV character witness the setting (showing), but sometimes, for the sake of brevity, slipping in a little omniscient right at the beginning establishes the important details of the setting (telling). Keep it brief.
Here's a fact: inexperienced writers tend to start their stories with way too much backstory. You, the author, need to know your character's childhood, education, political views, and job history because that drives the character's emotions, decisions, and actions. But the reader doesn't. Instead of having a paragraph or, heaven forbid, two pages of exposition telling the reader all that stuff, dribble significant items into the story when it matters. Do we need to know at the beginning of the story that Tom was in boy scouts twenty years ago? No. But if Tom gets lost on page 120, you could have him remember that less he learned at scout camp about finding north using the stars.
Explaining (In general, don't.)
Trust the reader. When I critique people's stories and chapters, I find myself repeating Trust the reader in my marginal comments. He opened the box to see what was inside. Well, we know why people open boxes. Don't explain why he opened the box.
Suppose Tom has been fleeing the bad guys...There was that noise again, just outside the window. Tom drew his gun in case one of the men who had been chasing him had found him. Why the explanation for drawing the gun. Tom drew his gun [period]. Trust the reader.
Tom swept her in his arms. He was overjoyed that she had come back to him. Well, duh. If you have to explain why he swept her in his arms, then you haven't set things up properly in the first place.
Wonderful Place was a monarchy. The ruler, King Braveheart, had ruled for twenty-five years. For the last three years, a great famine .... [and so on for two pages.] Please, no. See Providing Backstory, above.
Don't make parenthetical comments, or add an explanatory paragraph. The effect is the same as if the director of a stage show interrupted the performance by coming on stage saying, "Ladies and gentlemen, we'll pause here so I can point out to you that [blah, blah, blah]. Now back to the play.
Phony Artificial Suspense
Resist the urge to let the narrator add a comment at the end of a scene in a lazy attempt to generate tension.
If Jason had only known, he wouldn't have done that.
Grigor would come to regret letting his captive go.
Many years later, John would remember this night.
Grace walked on, unaware that the killer waited for her around the corner.
And that would be the last time Sam would see his children.
Cliffhangers are good, but the suspense should arise from the situation, and from the fears or thoughts of the characters. Avoid using a narrator's blatant attempt to generate tension.