Are you looking for a way to create conditional drop-down lists in forms where the options in a drop-down are dependent on the selections a person made in the previous drop-downs?
Quick example: If you offer a form on your site where a user first selects the year of their car, then the make, then the model from a list of options that only apply to the year/make that they already selected.
Not quite sure what cascading dropdowns are yet? Cascading dropdowns are typically used to help users filter data. The first dropdown is populated with the broadest options, and subsequent dropdowns are populated with options that relate to the selected value of the preceding dropdown.

Cascading dropdowns are great if you want to create forms to handle price estimates, quotes or payments on your WordPress site.
They’ve got a lot of advantages for the users too, because it saves them from having to filter through a ton of irrelevant options. You can import the options from CSV, so you don’t even have to enter all the options manually, making a form builder with cascading drop downs a big timesaver for everyone.