What are hierarchical fields in Tableau?

The hierarchical fields are those which have data arranged in hierarchies.

Hierarchies organize relevant data on different levels. We can dive deep into data and analyze it at a finer level by doing a drill-down into hierarchical fields.
Hierarchies arrange data fields in a level, for instance, a Geography hierarchy would have a region, country, state, city, area levels. Or, a Time hierarchy has a year, month, week, day as its levels. By creating hierarchies in Tableau, we set our data on different levels of detail and organize it.