When you create a lookup field on an object, you can choose whether the lookup field is required or is optional. If it is set as optional, you can choose one of the following three actions if the lookup record is deleted:
Clear the value of this field
Don't allow the deletion of the lookup record that's part of a lookup relationship
Delete this record as well
The clear the value of this field is the default option and is a good choice when the field does not have to contain a value from the associated lookup record.
The don't allow deletion of the lookup record that's part of a lookup relationship option prevents the lookup record from being deleted and is a good choice to restrict deletions if you have dependencies, such as workflow rules, based on the lookup relationship.