Its", is the possessive adjective and possessive pronoun form of the personal pronoun "it". "Its" means "belonging to it." For example, The cat licked its paw. On the other hand, "it's" is a contraction of "it is" or "it has". This simply means that we are combining two words (making a contraction) into one. So, instead of saying or writing for example; It is so nice to see you again, we could just have; It's so nice to see you again. The use of an apostrophe takes the place of the letter "i". This is the case when we use an apostrophe in combining two words to make it shorter. For example, she is = she's, does not = doesn't, I am = I'm, have not = haven't, and should not = shouldn't. As we can see here, we are replacing a letter with an apostrophe.
So remember, "it's" is short for "it is" (and less used "it has") and "its", is the possessive pronoun. Contractions always have an apostrophe, and is used to take the place of the dropped letter(s).