A more thorough explanation:

Marine risk, also known as peril of the sea, refers to the action of the elements at sea that can overcome the strength of a well-founded ship and the normal precautions of good marine practice. This can include extraordinary action of the wind and waves, collision, foundering, stranding, striking on rocks and icebergs, and even a swell from a passing ship. However, ordinary wear and tear are not included under the coverage of this or any other phrase in the clause, nor are losses which are anticipatable as regular incidents of sea carriage in general or of navigation in a particular part of the world.

For example, if a ship encounters a severe storm and is damaged or sunk as a result, this would be considered a peril of the sea. The carrier may be relieved from liability for the resulting losses. Another example would be if a ship collides with another vessel and is damaged, this would also be considered a peril of the sea.