dbt entities are typed join keys that link semantic models
dbt docs define entities as business concepts used as join keys—primary, unique, foreign, or natural—in the Semantic Layer.
Entities represent real-world concepts such as customers or transactions and act as join keys across semantic models in dbt’s Semantic Layer (v1.12+). Required parameters are name and type; types are primary, unique, foreign, and natural. Names must be unique within a model and may use expr; entities can also be used as dimensions.
Based on: Entities | dbt Developer Hub · dbt Labs