One of the most confusing aspects of English for many people is the distinction between "its" and "it's."
"It's" (with the apostrophe ) is a contraction for "it is":

It's a beautiful day.
It's hard to tell what you're thinking.
I looked in the cupboard but it's not there.

"Its" (without the apostrophe ) is the possessive form of the pronoun "it":

The dog had chewed through its leash.
We admired both the finish of the piano and its rich tone.
The award was presented to the club and its president.

One way to remember the distinction is to associate "its" with other possessive pronouns ending in "s": his, hers, and theirs.