Ansible Core is the command-line tool that is installed from either community repositories or the official Red Hat repositories for Ansible.

https://www.redhat.com/en/blog/ansible-101-ansible-beginners

With Ansible Automation Platform 2 release, few terminology changes were made.
One of those are, Ansible Engine as we know which included ansible binaries, modules are replaced with “Ansible-Core”.

Ansible Core is the foundational part of the Ansible Automation Platform.  It’s the command line tool, the language and framework that makes up the foundational content before you bring in your customized content.

The main differences between ansible-engine and ansible-core are “contents” e.g. modules and plugins.

Ansible Core only comes with limited number of contents.
(Number of ansible modules comparison between ansible 2.9 vs 2.11 can be found here.)

By moving contents out of the ansible-core, this provides following benefits:

• Agility
  – Currently ansible contents are being developed and managed by Open Source Communities, partners and Red Hat. Now modules can be updated and managed through developers-driven and ansible-independent schedules.
• A lean & Purpose driven execution environment
  – By only incorporating required plugins and modules, it bring the focus back into the users’ automation environment, rather than overloading with unnecessary contents.

As you can guess, this change was another foundation for Ansible Automation Execution Environment a.k.a ansible EE.