It is used to show possession or ownership.
Example : This is Sameer’s book. This sentence shows that Sameer is the owner of this book
Apostrophe is also used to join two words
Does + Not = Doesn’t.
Do + not = Don’t,
Could + not = couldn’t
Like conjunctions, it is used to join two sentences.
Read the instructions thoroughly; don't overlook any of them.
When in doubt, mumble; when in trouble, delegate.