Your vs You’re
These terms are not interchangeable. People often use them that way, but they’re not. One is a possessive pronoun, the other is a contraction of a pronoun and a verb. You don’t need to know that, just remember what an apostrophe is for.
When an apostrophe precedes an s, it indicates possession, such as: “That is Barry’s book,” meaning the book belongs to Barry When it does not proceed an s, it is taking the place of a letter. In the title example, it takes the place of an a. Written out, it would be you are. In the sentence: “Here is your pen,” your is a possessive pronoun, it indicates possession. You possess the pen, it belongs to you. To say, “Here is you’re pen,” would be saying, “Here is you are pen,” which just sounds silly.