It's means "it is". (它是)
Its is possessive (它的)
This is called a contraction. The [ ' ] (apostrophe) is where something was removed. Whenever you see the apostrophe ('), you know there is something there which was shortened.
Don't = do not
I'm = I am
Isn't = Is not
Aren't = Are not
Shouldn't = Should not
Couldn't = Could not
Ma'am = Madam
don't worry ... even native English speakers make mistakes! but now you can learn very clearly!
isler - be careful - "It's" should never be used for "It was". You cannot shorten "It was", only "It is". Actually, there is a contracted form of "It was", it is: [ 'twas ].
You can see this only in old songs, like [ 'Twas the night before Christmas ]
UPDATE - I should have said "you cannot shorten 'it has'". but the statement still stands; "it's" can only mean "it is"