JK, from Wikipedia:
"Per stirpes (pronounced /p?r ?st?rpi?z/ "by branch") is a legal term in Latin. An estate of a decedent is distributed per stirpes, if each branch of the family is to receive an equal share of an estate. When the heir in the first generation of a branch predeceased the decedent, the share that would have been given to the heir would be distributed among the heir's issues in equal shares. It may also be known as right of representation distribution, and differs from distribution per capita as members of the same generation may inherit different amounts."
Without that term, if your son predeceaces you, his share would go to your other remaining children (if any).
BTW, legal documents don't need to include terms that you don't readily understand. They can be written in English.
