Okay, so you know when you make a motorboat sound with your lips? (blblblblblb) That's called a "trill". Now, have you ever made a sound in the back of your throat by snoring or clearing your throat? That's where your epiglottis is. The epiglottis is like a tiny flap that covers the opening to your windpipe to help you swallow food or drink without choking.
A voiced epiglottal trill is when you make a sound by vibrating your epiglottis while also using your voice (that means you're making noise with your vocal cords). It's a really tricky sound to make and not many people can do it. It's kind of like making a motorboat trill, but instead of using your lips, you're using a different part of your throat. It's a fun party trick to show off, but it's definitely not something you should try at home without a lot of practice and guidance!