If an intent matches a request, the entity corresponds to the details. In booking a hotel room you need to know the exact date, the people who will stay, or the user's requests. From the design point of view of the agent, an entity such as hotel room is defined and it contains all the necessary details.
Dialogflow has a series of already-created system entities that facilitate the management of simpler concepts (for example, dates). The developer can define a series of entities (Dev entity) to generalize the behavior of the agent. Finally, the end user creates an entity for each request. A Dev entity can have values determined by a list, perform a mapping, or be in turn made up of other entities, and much more.
