This is a new site feature, designed to try to reduce some of the shortened word abbreviations used in posts, also known as "board slang," to make all posts more intelligible to everyone using the site.
In the words of one of our esteemed site users, Yazdzik:
If you have included certain abbreviated words in your post, usually more than once in that post, you will see the following screen when you go to review it:
The words that are flagged will appear in red. Simply return to your edit screen, fix these words and review again.
[Edit: right now, the target words are not appearing, so it's not easy to see which ones are hindering your post. Try just making sure all your words are spelled rather than abbreviated and that words like "gonna" aren't used.]