A dependent field is a hierarchical drop-down in a ticket. Dependent fields use a three-level hierarchy: Category → Subcategory → Item and each of these levels are represented by a drop-down box for customers. 

Please navigate to Admin > Workflows > Ticket Fields to add this field which is the last option on the "Drag and drop" toolbar. 

Here's how it works - when a customer chooses a category at the first level, a new drop-down appears with a corresponding set of subcategories. When they select a subcategory, they are shown a list of items in the third level based on their choice.