How do dependent fields and blocks work?

We will explain the functionality in a bottom-up approach with the following diagram:
   

1. Firstly, the controller (Trigger) field is activated. 
2. Next, the specified conditions are checked and matched with the Trigger field.
3. Lastly, based on the access rules, the Dependent Fields and Blocks are shown. 

Consider this example.
A customer sells insurance with Product A and Product B. He has 120 custom fields for Product A and 120 custom fields for Product B. The customer’s requirement is straightforward.  They don’t want Product B’s fields to be visible while working on Product A and vice-versa.
Here is the solution.
With the help of Dependent Fields and Blocks, we can achieve the above requirement. When the customer selects Product A from a Picklist or Checkbox field, the fields related only to Product A are shown, and when he selects Product B, the fields related only to Product B are shown.

To know more about Dependent Fields and Blocks, click here.