Definition
A knowledge panel is the information box Google shows for an entity it knows about: a person, organisation, place or work held in its Knowledge Graph. Google generates it automatically from sources across the web, which is the fact everything else follows from.
Example. Search the name of a well known author and Google may show a box with their photo, books and key dates. Nobody at the publisher wrote that box: Google assembled it.
You can check whether Google already treats your brand as a recognised entity, panel included, with the free Entity SEO Auditor skill.
Can you create a knowledge panel?
You cannot create a knowledge panel. Google states that knowledge panels are generated automatically, and shown for entities that are in its Knowledge Graph. What you can do is claim one that already exists, if you are its subject or an official representative, and then suggest changes to it. Anyone else can suggest an edit through the feedback link on the panel.
Is a knowledge panel the same as a Business Profile?
A knowledge panel is not a Business Profile. Google treats them as separate features: a Business Profile is for a business that serves customers at a location, and it is created and claimed by the business. A knowledge panel is about an entity in general and is assembled by Google. A local shop can have a Business Profile and no knowledge panel at all.
Where does a knowledge panel get its facts?
A knowledge panel gets its facts from various sources across the web, plus data partners on some topics, according to Google, and it updates automatically as that information changes. That puts the real lever upstream of the panel. The facts published about you across the web, on your own site and everywhere else, are what the box is built from.
That is also why a claimed panel can still drift. Suggesting a change fixes the panel today, while the sources it draws on decide what it says next month.