Base fields are the fields most close to a given entity type, things like the title, creation/modification date, publication status, and so on. They are defined in the entity type class as BaseFieldDefinition implementations and are installed in the database based on these definitions. Once installed, they are no longer configurable from a storage point of view from the UI (except in some cases, in which certain aspects can be overridden). Additionally, some display and form widget configuration changes can still be made (also depending on whether the individual definitions allow this).
Let's check out the Node entity type's baseFieldDefinitions() method and see an example of a base field definition:
$fields['title'] = BaseFieldDefinition::create('string') ...
