It's supposed to be a reminder - I'll explain: If you've studied other languages, some of them have quite complicated vowel systems, like English. But many of them, like Spanish, Italian, Japanese, Hawaiian, etc. have a very simple 5-vowel system. Klingon also has a very simple 5-vowel system which is very similar to those 5-vowel systems with one big difference. The a, e, o, and u in Klingon are pronounced pretty much exactly the same as in those languages. But all of those languages pronounce the "i" like "ee" as in the Spanish "amigo" (pronounced ah-mee-goh). This is where Klingon is different. The I is pronounced like the "ih" sound from "tin". The Klingon word tIn ("to be big") sounds exactly like the English word "tin". Dr. Okrand decided to capitalize the I as a way to help people remember that the pronunciation may not be what you expect. It turns out to be more confusing than helpful, but now all the materials in and about Klingon use the same transcription system, so we teach this system too.
If you look carefully you can see a small curl at the bottom of the l (lowercase L) that is absent from the I (uppercase i). I wish we had the option to use a font that makes the difference more clear. As you get used to the language you'll also get better at naturally seeing where the letter must be a consonant or must be a vowel. Experienced speakers rarely have any difficulty telling whether it is I or l based on placement in the word.
As a beginning to understand the difference, all consonants must have at least one vowel next to them. In the word from this exercise there is no doubt that the t is a consonant. If the letter next to it is a lowercase L, then there is no vowel next to the t (or the L either). The letter next to the t must be an uppercase i.
While we're discussing the word 'It I would also like to point out the importance of the letter ' (called qaghwI'). It is not a punctuation mark - in Klingon it is a full letter of the alphabet and leaving it off or putting it in the wrong place is a spelling error. The qaghwI' is also a consonant. So if the word were 'LT, there would be no vowel at all, so the letter between ' and t must be the vowel I.