A roll-up summary field calculates values from a set of related records, such as those in a related list. You can create a roll-up summary field on the object that is on the master side of a master-detail relationship, such as the Order object, and summarize the values of a field in the detail records, such as the Order Line items. You can choose the SUM roll-up type to get the total of all Order Line item totals on the Order record. This is the best practice because it does not require any code or additional actions to maintain the accuracy of the field value. References: Implement Roll-Up Summary Fields Unit | Salesforce Trailhead, Roll-Up Summary Field - Salesforce, How to create Roll-Up Summary fields in Salesforce