Translation:the letter ch
Yes -- b d g v z th are voiced, and p t k f s th are voiceless (and p t k are, in addition, usually aspirated).
Voiceless and voiced "th" are (unfortunately?) spelled the same in English -- "thigh" and "thy" are distinguished in pronunciation only by the voicing of the initial sound, but you can't tell from the spelling that "thigh" starts with a voiceless sound, "thy" with a voiced one.
That should have been "lenis" I misspelled it with a "t". English also has fortis/lenis pronunciation, but we don't consider it to be distinguishing - for us, it is much more important whether you voice the consonant or not. I have not studied any German in depth, but from what I understand, the minimal pairs are usually distinguished by strength more than by voicing. Here's a wikipedia page that describes it: https://en.wikipedia.org/wiki/Fortis_and_lenis
Have you read the Tips & Notes which explain the pronunciation of each of the letters? 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 continuously repeat 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 (or in this case, a brief explanation of how to pronounce each letter). 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.