A thimble is used with rope when splicing the end of a rope to form a hard eye or loop that can be connected to shackles etc.  Usually a heart shaped piece of steel insert that is used to preserve the natural shape of the loop from pinching and abrasion.  Available in commercial zinc plated (galvanised) or stainless steel.  You can also use with galvanised wire rope where a length is required to be terminated into a hard eye.