The punctuation mark (‘) is called apostrophe. It has the following functions.
In writing a contraction
The apostrophe is used in writing a contraction. It shows the place of the omitted letters. Examples are: it’s (for it is or it has), hasn’t (for has not) and doesn’t (for does not).
The apostrophe is also used in writing certain words which were formerly contractions. Example: o’clock
In writing most possessives
The apostrophe is used in writing most possessives.
Children’s books two weeks’ vacation
Note that the apostrophe is not used in writing the possessive forms of most pronouns.
The cat closed its eyes. (NOT The cat closed it’s eyes.)
Whose bag is this? (NOT Who’se bag is this?)
These toys are hers. (NOT These toys are her’s.)
In writing the plurals of letters
Although the apostrophe is not used in writing plurals, it is used in writing the plurals of letters and other forms which would be difficult to read without the apostrophe.
Dot the i’s and cross the t’s
Mind your p’s and q’s