Its is the possessive form. This is one of the few possessive forms of a word in which the apostrophe is deliberately omitted. This is to distinguish it from the abbreviated version of the expression it is shortened to it’s. Examples:
It’s awful (it is awful).
The dog bit its owner (possessive form of “it”).
It is important you do not mix up these two spellings, as editors get IRRITATED at having to correct the error. Editors feel it is so basic that all freelancers should get it right all the time.
Just to make matters more confusing, don’t forget that the possessive of her, your and their is spelt without an apostrophe: hers, yours and theirs (because the words themselves are possessive). There is no such word as its’.