Translation:I can read it.
Some letters in Klingon are always lowercase: a b ch e gh j l m n ng o p r t tlh u v w y. They're lowercase even at the beginning of a sentence or at the beginning of a person's name.
Some are always uppercase: D H I S. They're uppercase even in the middle or at the end of a word.
(This also means that if you see the sequence ngh in a word, it has to be n + gh and cannot be ng + h, because lowercase h isn't a letter in Klingon.)
And then there are the two letters q and Q, which are separate letters in Klingon, though in English they differ only by capitalisation. And the apostrophe ' which is a letter in Klingon but doesn't have a case in English.
Two letters to pay particular attention to are l (small ell) versus I (capital eye). The small L should have a little curl at the bottom in Duolingo's font, while the capital i should be completely straight.
The prefixes jI- and vI- both have a capital i, not a small L.
jI- indicates that the subject is "I" and that there is no object.
vI- indicates that the subject is "I" and that the object is one of "him, her, it, them".
So jIlaDlaH. is "I can read." while vIlaDlaH. is "I can read it." or "I can read them."
This information is explained in the Tips & Notes, which makes me wonder if you've found them yet. I know it’s a lot of new information and some details are bound to slip through the memory. However, since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. If you have not been reading the Tips & Notes, I would like to ask that you review those so we don’t have to repeat too much of the information that we have explained there.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask your question, explaining what you didn’t understand or what seems contradictory to you.
where is the 'it' on the sentence ?
In the prefix.
vI- is a prefix that is used when the subject is first person singular ("I") and the object is third person ("him / her / it / them").
Since you usually don't read people, vIlaDlaH. most reasonably translates to "I can read it." or "I can read them." (e.g. the books).
The sentence "I can read." would be jIlaDlaH. with the jI- prefix that indicates that the subject is "I" and that there is no object.