A roll-up summary field calculates values from related records, such as those in a linked list. The roll-up summary is a gathering calculation that pulls information from several areas. You field to display a value in a master record based on the values in a detailed record. The detail record must be related to master through a master-detail relationship.
You can execute different calculations with roll-up summary fields. You can count the how many detail records related to a master record. Roll up summary fields and fields are always read-only on detail pages and are available on edit pages. You can create a roll-up summary field to display a value in a master record based on the values of fields in a detailed record.
Because roll-up summary fields are not displayed on edit pages, you can use them in validation rules but not as the error location for you validation. Validation errors can display when saving either the detail or master record. Once created, you cannot change the detail object selected or delete any field referenced in your roll-up summary definition.