This section of Sekiro: Shadows Die Twice includes the description of the Grappling Hook - one of the prosthetic tools available in the game. You will also learn how to unlock and then use the Grappling Hook.
How to unlock the grappling hook?
You receive the grappling hook automatically after completing the prologue - the first time you meet the Sculptor at the Dilapidated Temple. The game will grant you access to this tool after you leave the temple.
Using Grappling Hook
The most important function of the grappling hook is the ability to use interactive grapples. You cannot reach any location through the in-game world, as grapples appear only in selected areas.
A grapple can be a tree limb, a roof, a ledge, etc. Upon approaching a grapple, you will notice a grey icon. You can use your grappling hook after this icon turns green (example in the screenshot).
Some of the interactive grapples are less accessible and might require you to leap and hang on to the grapple in midair (an example in the screenshot). Make sure not to fall when doing so.
Later on, you can unlock Grappling Hook for combat purposes by unlocking the Grappling Hook Attack from the Prosthetic Arts. You still have to lock on interactive grapples around you. The difference lies in the fact that you can perform a Katana attack when pulling yourself towards the selected grapple. This is a good way to eliminate enemies standing e.g. by the ledges.