There is a parent-child (ownership) relationship between tblEmployee and tblPaycheck. Employees may have lots of paychecks. Brand new employees won't have any paychecks (yet). Every paycheck, however, must be owned by exactly one employee. If a paycheck can be made out to an unknown person, then we clearly have a terrible payroll system. Someone in the Accounting Office is about to lose their job over the orphan paycheck (i.e., no parent employee).
There is a parent-child (ownership) relationship between tblStudent and tblGrade. Students may have lots of grades. Brand new students won't have any grades (yet). Every grade, however, must be associated with a single student. If a grade can be assigned to an unknown person, then we have a terrible university database. Someone in the Registrar's Office is about to lose their job over the orphan grade (i.e., no parent student).
There is a parent-child (ownership?) relationship between tblWoman and tblKid. Women may give birth to lots of kids. Women don't have to have any kids. Every kid, however, has (or had) a biological mother.
What is the difference between paychecks, grades and kids?
An orphan paycheck means we have a seriously flawed payroll system. An orphan grade means we have a flawed university database. An orphan kid simply means their parent died. Orphan kids are perfectly acceptable. Orphan paychecks and grades are not.
A paycheck that is made out to to a non-employee is a major problem. A grade assigned to a non-student is a major problem. A kid who belongs to a non-parent is a major problem.
Referential integrity is the term that means we want to prevent a child record from pointing to an unknown parent record. A child record may not have a parent, but if it is does have a parent, then it must be a "real" parent. The designer of a database must decide which relationships should enforce referential integrity. The designer must then decide which relationships should allow orphans and which ones should not.