Roll-Up Summary Field

Create a roll-up summary field to display a value in a parent record based on the values of fields in a detail record.

This is applicable for master-detail relationship only.

For Example :

To display the sum of proposal amounts for all related proposal(child) custom object records in an Account’s (parent) proposal related list.

This total can be displayed in a custom account field called Total Proposal Amount.

Details -  https://www.youtube.com/watch?v=z6lI_ZO4SmE

Reference: https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5