What is?
https://www.drupal.org/docs/drupal-apis/entity-api/working-with-the-ent…
Often when building a site in Drupal you'll find yourself wanting to display a list of nodes, or find entities created by a particular author, or locate some content based on a particular set of criteria.
How to use?
In Drupal, you can use the Entity Type Manager service in various types of PHP files, primarily within custom and contributed modules or themes to work with entities.