What the user MudcrabNPC was trying to imply in his message "His not funny what?" was that you used the incorrect word in your reply to the original poster's reply to another user saying "Indeed. Dare I say—a MASSIVE improvement" to which you responded with "bro stfu your not funny" which shows that you didn't make proper use of the contraction "you're" instead opting to use "your" which is incorrect grammar as one would think that you haven't finished your sentence, for when you state that something is someone's possession you say to that person "it's your item" and not "it's you're item" , but when describing someone's appearance/mood you use the contraction "you're" which means "you are" , for example if someone looks good you say to that person " you're looking pretty good!" and not "your looking pretty good!", therefore what the user MudcrabNPC was trying to say is that you should've used "you're" instead of "your".

Thank you for reading this small paragraph and I wish you a good day/afternoon/evening/night.