Stupid sound question - "DNS Error"

It sounds like an obvious answer, but what does the http/https monitoring error "DNS Error" mean, exactly?

Specifically, I have several https checks defined in Hobbit, and a good number of them get the "DNS Error" message multiple times a day; roughly on the order of every 1-3 hours. I have manually verified that DNS is working for these hosts. It might help me troubleshoot this if I know exactly how Hobbit comes to this determination, and if anything else other than DNS could cause it to appear.