The last kiss is a rite practiced by the red priests of R'hllor. When a follower of the Lord of Light dies, priests fill their mouths with fire and breathe flame into the deceased, as they believe that fire cleanses and is a bright gift. Harwin and Thoros of the brotherhood without banners refer to it as the kiss of life.
A Storm of Swords
Thoros of Myr explains to Arya Stark that he administered the last kiss to Lord Beric Dondarrion when Beric died after the battle at the Mummer's Ford. The kiss revived Beric, however, much to the shock of Thoros, who had never seen such an occurrence before and attributes the resurrection to R'hllor.
A Feast for Crows
Thoros tells Brienne of Tarth that when the brotherhood without banners found the long-dead body of Catelyn Stark, he refused to perform the kiss of life on her. Beric instead touched his lips to Catelyn's, passing the flame of life from him to her.