Retard text syndrome is when you're texting someone and they do not know how to text. It's when they say "u" instead of "you" and "r" and so on. It's when they can't spell. They say "lyke" instead of "like" or "hym" and many others.
John: "Aye bro. Y didnt u com 2 tha prty? Lyke evry1 wuz ther."
Bob: "Dude. Don't talk to me until you get that retard text syndrome checked out."