The omniscient voice is when the story is told by an all-seeing, all-knowing narrator. It's also called the authorial voice. The narrator knows what's going to happen in the future, what each character is thinking, what the implications are of actions and so on. 19th century novels use authorial voice a lot - "it is a truth universally acknowledged..." or "All happy families resemble each other, but each unhappy family is unhappy in its own way" can only be said by an omniscient narrator.
It can be a distancing voice, putting us at one remove from being inside the heads of the characters directly ourselves. But it also has its uses. I was re-reading The Hobbit, and was struck by how JRR Tolkien's omniscient voice added to the telling of the tale. It told where they made mistakes - "actually, as I have told you, they were not far off the edge of the forest; if Bilbo had had the sense to see it", poked gentle fun at Bilbo and his fondness for home comforts and food, went into amusing digressions such as how the game of golf was invented and filled us in on what was happening when Bilbo wasn't actually present.
So, the omniscient voice isn't wrong. For the right book it's just right.