What is Roll up summary field 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. For example, you want to display the sum of invoice amounts for all related invoice custom object records in an account’s Invoices related list. You can display this total in a custom account field called Total Invoice Amount.