Welcome back from Labor Day! To get us all back in the writing mood, here's a quick reminder of everyone's least favorite grammar mistake.
It's is the contraction form of "it" and "is".
Its is the possessive form (meaning, the pronoun "it" owns the following object noun) of "it" (like "his" is the possessive form of "he").
To check, ask yourself, can I separate this out into "it" and "is"? If you can, then you need the apostrophe. I remember this by pointing out that pronoun possessives usually don't take an apostrophe (his, her, my, your, our), but that contractions do (we'd, I've, she'll).