"They are targs."
They are both plural pronouns, but bIH refers to a collection of things which do not include the speaker or the listener and are not beings capable of language. It is usually translated as "they" or "them". Because it is being used like a verb here it is translated as "they are".
tlhIH refers to a collection which includes the listener, but not the speaker. It is usually translated as "you", (it is the plural form of SoH), because English does not distinguish singular "you" from plural "you", but Klingon does. tlhIH is not restricted in whether the listener is a being capable of language or not, but it would be unusual to be speaking to something that was not.
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, 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.