Klingon uses the prefixes as a conjugation system to show who is doing the action, but also who the action is done to. In other words, the prefixes tell you the grammatical subject AND the grammatical object. The jI- prefix tells us the the person speaking is doing the action, but it is not being done to any particular object. So jIyaj means, "I understand". The vI- prefix also means that "I" am doing the action, but in addition indicates that there is an object and it is third person (not "you" or "me"). So, vIyaj means, "I understand it/him/her/them." (In conversation, context will usually clarify whether you should translate it as it, him, her, or them. Duolingo does not usually provide any context so it accepts each of those four answers.)
Similarly, bI- means, "you (singular) - no object", ju- means, "you (singular) - us", and cho- means, "you (singular) - me". Though Duolingo has not made the Notes available on the app, there are grammar explanations available for you. If you have not been reading the Tips & Notes, I would like to ask that you review those.
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 large Start button, a small key button, and a large Tips button (or a small light bulb button - the small light bulb button is pictured here).
If you click on the Tips button (or the light bulb if you have that instead) 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, we are happy to answer your questions, but many of your questions will probably already be answered in the Tips & Notes.