A more thorough explanation:

Definition: Forward and backward at sea refers to the movement of a ship from one port to another during a voyage, and not just between two endpoints.

Example: A ship carrying cargo from New York to London will travel forward and backward at sea. It will not just go from New York to London and back, but will also stop at other ports along the way.

Explanation: When a ship travels forward and backward at sea, it means that it is making stops at various ports during its voyage. This is important in marine insurance because it affects the risk and potential losses that may occur during the voyage.