Writers are sometimes not sure about when to write it’s or its.

An apostrophe means that a letter has been left out (sometimes more than one letter).
It’s is the abbreviated form of it is (or sometimes it has).

It’s hard to believe that he is 14 years old (It is hard to believe…).
The book is very thick, but it’s really interesting (it is really interesting).
It’s been a long day (It has been a long day).
It’s got to be true (It has got to be true).

Its is the possessive form of the pronoun it.

Stockholm is known for its many islands.
The cat was licking its paw.

The cat was licking its paw

The simple rule is this: If you can say it is or it has, then the form with an apostrophe, it’s, is correct.

It’s frightening when the bear opens its massive jaws

The abbreviated form it’s should not be used in formal language – there you should write it is.

What is true about it’s and its also applies to you’re and your, they’re and their or who’s and whose.