In the Fate universe, the "Saber heart" represents the core of the King of Knights, Artoria Pendragon . Her story is often defined by whether or not her master, Shirou Emiya , can "save her heart" by teaching her that it is okay to be human rather than just a perfect king.
Saber's Wish Chapter 1: A Fateful Birthday, a fate/stay night fanfic saber heart
The rain in Fuyuki City didn't wash away the scent of ozone and blood; it only made it heavy. Artoria—the Servant known as Saber—stood in the middle of a shattered courtyard, her invisible blade trembling in a way a legend never should. In the Fate universe, the "Saber heart" represents