What is the use of a Roll-Up Summary Field and where it can be used in Salesforce?

A roll-up summary field calculates values from related records, such as those in a related list. You can create a roll-up summary field to display a value in a master record based on the values of fields in a detail record. The detail record must be related to the master through a master-detail relationship.

Please check this link, it has the explaination-  https://help.salesforce.com/articleView?id=fields_about_roll_up_summary_fields.htm&type=5