"Per stirpes" is a Latin term translated as "by the roots or by representation." In estate planning, the term is used to denote a method of dividing the estate of a person. A person who takes per stirpes, sometimes called by right of representation, does not inherit in an individual capacity but as a member of a group.
In order to understand use of this more clearly, describing a situation where it applies can help. Let's say Grandma specifies in her Will that her estate is devised "to my two children, in equal shares per stirpes." If Grandma dies and both children are living, then Grandma's estate would go to the two children equally. But what if one of Grandma's children has predeceased her? If the child who died had two children, then Grandma's estate would be distributed 50% to the living child and 25% to each of the grandchildren. On the other hand, if the child who died had no children or grandchildren, then the estate would go 100% to the living child (unless the Will specified otherwise).