Any object that is on the master side of a master-detail relationship.
A roll-up summary field can be created to show a value from the master record that is based on field values from a detail record. However, to accomplish this, the detail record should have a master-detail relationship with the master, in order to calculate the values from the related records.
One example would be if you want to display the total sum of all invoice amounts for every related custom invoice records in an invoice account list. This can be accomplished by creating a roll-up summary field that is called “Total Invoice Amount.”