What does the apostrophe after the word mean?

An apostrophe used after the letter S at the end of the Word generally means it is a plural possessive, such as "cats' meow" which is basically referring the meow to all of the cats (plural).

Sometimes, however, some Words will end in an S without being a plural, such as names, but the apostrophe still indicates that it is possessive. For example "James' notebook" which is saying that the notebook belongs to a singular James.