It does matter. They are two separate words with two completely different meanings, and it is straightforward to decide which to use.
You're is short for YOU ARE.
(So, for example, "you're looking very well.")
Your refers to belonging to, or associated with a person.
(So, for example, "your coat is in the other room.")
If you're not sure, just ask yourself if you are trying to say YOU ARE, or not!