In First Person Narration, the narrator is also a character/cast in the story.
Nick Carraway in "The Great Gatsby";
Scout in "To Kill a Mockingbird";
and of course, Dr. Watson of "Sherlock Holmes".
In Third Person Narration, the narrator is an outside the story. You can know that the story is in the Third Person when the narrator uses pronouns in third persons like he, they, she.
Example is "The Harry Potter' series where the point of view narration of the third person is limited to Harry Potter.