Translation:I killed Kahless.
English and many other languages only change the verb based on the subject (the one doing the action), but Klingon prefixes also include information about the object (who or what the action is done to). jI- means that an object is not being specified: I walk, I sleep, I eat, I read, etc. Some of these verbs can take an object, but no object had been included in those sentences, so jI- would be the proper prefix. jIHoH means "I kill," and makes no mention of who or what you kill.
vI-, on the other hand, means that a grammatical object IS being specified. I may be assuming you know the grammatical object I'm referring to or I may actual add the object to my Sentence: I eat it, I read books, I see her, I drink water, etc. vI- would be the proper prefix with a first-person singular subject ("I") and a third person object ("he/she/it/them"). vIHoH means "I kill him/her/it/them." Without specifying who him/her/it/them is/are I'm leaving some vagueness, but usually there is context which would allow you to understand exactly who or what I meant. In this case the name of Kahless has been added, so you know exactly who is being (or has been or will be) killed. Because there is a grammatical object (qeylIS), you cannot use jI-.
An explanation is given in the Tips & Notes, but I know it’s a lot of new information and some details are bound to be difficult to understand or 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 and a Tips button.
If you click on the Tips button 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.