The wordit's is short for it is. We use the apostrophe to show that a letter is missing. Here is an example that shows how to use it's. It's very hot today. (It is very hot today.)
The wordits means belonging to it. This is similar to the word her (belonging to her) and his (belonging to him). Here is an example that shows how to use its. Look at that sheep – its wool is very long.