Entities are content types in Sitecore Content Hub. Examples includes Asset, Product or Project.
The Entity Definitions management page allows users to create new Entity definitions, edit or delete existing ones.
Changing Entity definitions changes the structure of the data. Therefore, the impact of these changes can't be underestimated and all changes to existing entity definitions should be performed with utmost caution. Deletion of entity definitions is only possible when no relations or taxonomies are linked to the definition itself, same as modifying this has repercussion on the structure of data and needs to be performed with utmost caution. Additionally all entities created under this entity definition will be lost.