In the context I use the word it means personal servant. If I understand it correctly their job was to wait up to their employer and keep clean in his private rooms. It was also them which nursed him when he was ill. Please note that the word is only used about men. A female personal servant is instead called chambermaid. I think men had male personal servants and women female ones. The last six years of his life Napoléon had three valets. They were named Louis Joseph Marchand, Jean Abram Noverraz and Étienne Saint-Dennis.