> Pick any topic you want: a hobby you have, a book you’ve read, a project you worked on–anything. Hopefully there's more guidance than this, because I feel like I'd freeze up and start "um"ing for five minutes trying to decide the best concept to explain. Maybe this says more about me than the exercise, though...
Now, to the point about you: you should practice this. Being good at talking about your hobbies and interests in technical detail while explaining clearly is an extremely valuable skill. Learn it.