We Raced To Circumnavigate The Globe In 100 Hours

This might be a dumb question, but what is a charter flight?
Google returns only the company Stratos making articles about themselves and how it is amazing to "book a flight for yourself" and "basically rent an entire plane".
This might be all well and good. However I don't think they have the budget to buy charter flights. I mean, only the fuel for an hour of flight costs tens of thousands of dollars.