Third-person limited refers to a character in the story that provides the reader with inside thoughts of ONLY ONE character but not the thoughts of other characters. Example: “John was pained to see Lisa coming around the corner. He worried that she might still want to get back together.” In this example, the reader knows about John’s feelings and thoughts, but NOT about Lisa’s.
Third-person omniscient refers to a narrator that is NOT a character, but is all-knowing and is able to share the background story and/or inner thoughts of ANY character. Example: “John ran into Lisa at the convenience store. He mistook her good humor and friendliness as a desire to get back together with him. Lisa smiled at John, happy to tell him that she was newly engaged.” In this example, the reader is privy to the thoughts of BOTH John and Lisa.
Hope that clears up any confusion for you!