Ketjap manis is a thick, sweet, rich, syrupy Indonesian version of soy sauce containing sugar and spices. It's similar to soy sauce but sweeter. It's used in marinades, as a condiment or as an ingredient in Indonesian cooking. The sweetness comes from palm sugar; other flavourings include garlic and star anise.
Look for ketjap manis in specialist oriental stores. Chinese and Japanese soy sauces are not in themselves suitable substitutes; if they were used to replace ketjap manis, the dish would need additional sweetening.