You can search for a condition like back pain or chickenpox, for instance, and read a short description for it, a list of symptoms and a list of possible treatments without having to click through to visit a website. Google says the Knowledge Panels will be available for over 250 conditions to start with, so the feature will presumably expand to cover even more.
The company uses Knowledge Panels to highlight results from reputable sources for various topics, like celebrities. Searching for a musician's name will bring up Knowledge Panels with information about them and their work, as well as links to where you can hear their music and their videos. More recently, it started displaying Knowledge Panels for COVID-19 when you search for coronavirus, which contain details about the condition from the World Health Organization.