You may not realise but you already saw one of these:

A PHPMyAdmin ERD

They are diagrams which show the relationship between entities in a database.  A more simple form is this:

A proper ERD

You will need to draw these if you ever design a database properly.  The PHPMyAdmin version is more helpful as it shows which fields are used to create the relationship.  The simpler version lets you grasp the overall picture more easily (important as some databases may have tens or hundreds of related tables).

Note that the proper ERD uses a 1 and an infinity symbol to represent the one-to-many relationship.  PHPMyadmin uses a dot and a semi-circle.  You may also see a branching arrow at the many end instead.