As an omniscient narrator, you float over the landscape wherever you want, moving from place to place in the twinkling of an eye. You pull the reader along with you like Superman taking Lois Lane out for a flight, and whenever you see something interesting, you explain to the reader exactly what's going on. You can show the reader every character's thoughts, dreams, memories, and desires; you can let the reader see any moment of the past or future.
The limited third-person narrator, on the other hand, doesn't fly freely over the landscape. Instead, the limited narrator is led through the story by one character, seeing only what that character sees; aware of what that character (the "viewpoint character") thinks and wants and remembers, but unable to do more than guess at any other character's inner life. You can switch viewpoint characters from time to time, but trading viewpoints requires a clear division-a chapter break or a line space. The limited third-person narrator can never change viewpoints in mid-scene.