Belacan is a salty, concentrated fermented shrimp paste used in Malaysian and related Southeast Asian cuisines. It often comes in a firm block and is cooked or toasted as part of making a seasoning paste, sambal or other dish.
A small amount supplies a strong savory flavor. It contains shellfish and differs in moisture and strength from other shrimp pastes, so substitutions need adjustment. It is not vegetarian despite the small amount usually used.
Reference: Further reading.