It’s incredibly frustrating that only the first letter is highlighted. I was guessing tons of SU… and SV… words, but only the S had visual feedback. Bug?
It's a hint to tell you the word starts with "s", but since you didn't narrow it down to "su..." or "sv..." it's not giving you more hints.
Once you narrow it down further, say, "sub..." and "sun..." it'll highlight the known prefix "su"