Final answer:
An apostrophe has two functions: indicating possession and forming contractions.
Step-by-step explanation:
An apostrophe has two functions. It indicates possession and forms contractions. When used to indicate possession, an apostrophe and -s are added to a singular noun or indefinite pronoun. For example, 'Jack's brother is my sister's coworker.' Additionally, when forming contractions, an apostrophe is used to show the omission of one or more letters. An example of a contraction is 'It's raining today,' which is short for 'It is raining.'