Advanced Element Queries - example uses
These are simply low level operations you can do to more specifically filter the elements that should be returned. It is generally more common to use them in plugins/modules than in Twig templates, but the use-cases are numerous. A few trivial examples are below.
{{ set entries = craft.entries.where(['authorId', 1]).all() }}

{{ set entries = craft.entries.where(['in', 'authorId', [1, 2, 3]]).all() }}

{{ set entries = craft.entries.where(['&gt;=', 'dateCreated', '2021-01-01']).all() }}

{{ set entries = craft.entries.where(['like', 'title', 'prefix%']).all() }}

Detailed docs for where and other DB query parameters can be found here:
https://www.yiiframework.com/doc/guide/2.0/en/db-query-builder#where