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.