Rollup summary fields 

A rollup summary field (RUS) field is very much like a formula field in that it also performs a calculation. However, this is based on child records, and only when these child records are part of a Master-Detail relationship!

It's used to summarize a numeric value based on (filtered) child records and can perform COUNT, SUM, MIN, and MAX.

It is recalculated whenever a transaction on these child records occurs in the following ways:

• When a new child is created
• When one of the children gets updated
• When a child gets deleted

You can use filters to only pull values from specific child records. For example, let's say that we have a custom Invoice__c object that is related to the account as a Master-Detail relationship. In our Invoice__c object, we have an Amount field and a Status field. We could create a RUS field in the account that represents the total amount that is overdue, which is a calculation of the SUM operation of the Amount field on all related invoices, with a filter on the Status field that is equal to Overdue.