Query is a term used to describe the act of selecting, inserting, or updating data in a database. In WordPress, queries are used to access data from your MySQL database. WordPress is written using PHP and MySQL.
Each time you are viewing a WordPress page, there are MySQL queries running in the background to fetch the data from database. This data is then used to dynamically generate HTML for your browser. When users create, edit, or delete anything from WordPress, there are database queries that convert user input into instructions which are then executed by running database queries.