In royal families, the crown goes to the oldest child. That's the heir. The "spare heir" is the next child who would ascend the throne should something happen to the heir. In the UK, when William becomes King, Prince George is the heir. His brother or sister, who will be born in April, will be William's spare heir. Harry was the spare heir until Prince George was born.

　　在王室中，王位传给长子，称作王位继承人。如果王位继承人发生不测，下一个继承王位的人叫做备胎继承人。在英国，当威廉王子成为国王时，乔治王子就是王位继承人，而即将在4月出世的二胎，无论男女，都会成为威廉的备胎继承人。在乔治王子出生之前，威廉的弟弟哈里王子一直是备胎继承人。

　　For example：Aristocratic wives were once told they needed to provide an "heir and a spare heir" at least。

　　贵族的妻子们曾经被教导要至少生一个继承人和一个备胎继承人。