Possessive nouns modify other nouns to indicate possession.
Peter's motorcycle
The building's structure
For nouns ending in "s," or to use the possessive with regular plurals, place the apostrophe directly after the "s." Do not add another "s."
Parents > parents' concern for their children
Computers > computers' manufacturer
Notice that this construction can change the meaning from singular to plural.
The cat's favorite food is tuna. (one cat)
The cats' favorite food is tuna. (more than one cat)