This basically means a person is ignorant. It's based of a fable in which a turtle from the Eastern Sea came ashore and was called by a frog inside a well.The frog boasted about his knowledge of the well and wanted the turtle to visit him.The turtle however,couldn't climb over to reach the frog and told the frog that his views are limited to what he knows and because the frog was basically confined to the well,he couldn't know the beauty and vastness of the world around him.