In Daniel's Story, why did Peter say remember as his last words? What did he mean by that?

The importance of remembering is emphasized at several places in the novel. This message fits with the idea of the ongoing necessity of testimony to the horrors of the Holocaust and the tremendous acts of resistance, both as a way of healing for survivors and their families and as a path toward preventing that anything similar ever again transpires. It is an appropriate message for Uncle Peter given his profession of photographer, as he was in the business of preserving memories through images. Daniel as well thinks back that Erika had advised him to remember that those who perished had been destroyed by hate, which represents the worst qualities of humanity, and that whenever they were faced with a choice, the survivors instead should opt for love.