You mean the Latin “per stirpes”. “Per” = through, by means of.
“Stirps” (singular) “stirpes” (plural) = stock or branch, descendant.
Often we find in wills (in lieu of “per stirpes”) the use of the phrase “by right of representation”, but meaning the same thing.
For example, say that a will provides that certain assets of the maker’s estate are to be distributed equally to the testator’s named children,“per stirpes”.
If the one of the children should predecease the maker of the will (and it remains unchanged), then that deceased child’s children (if any) are to share equally in the portion of the estate that would have been passed to the parent if living.
It is important to note that per stirpes doesn’t mean that the grandchildren share equally with surviving uncles and aunts.
(All of which is no doubt more than you care to know about the boring subject.)