Prince Caspian Narrator:
Hey! Watch the language, buddy. This is a children's story. Don't go calling people omniscient.
First, an example is in order:
So the Dwarf settled down and told his tale. I shall not give it to you in his words, putting in all the children's questions and interruptions, because it would take too long and be confusing, and, even so, it would leave out some points that the children only heard later. But the gist of the story, as they knew it in the end, was as follows. (3.57)
We can tell this is a third-person narrator because it's clearly not someone in the story. There's no "I" character saying "I did this" or "I did that." Instead, it's all "the Dwarf did this" or "he did that."
The omniscient part simply means the narrator can access information that the in-story characters cannot. For example, he's able to tell us the complete story of Caspian well before any of the Pevensie children or even Trumpkin, the character currently telling the tale. That makes the narrator omniscient—or "all knowing" if you prefer a little less Latin in your daily wording.
See? Omniscient third-person narrator may sound like quite a nasty thing to call someone, but it's not so bad.