From Halopedia, the Halo wiki
Havadi Goodwan is a human restaurant chain on Earth and Reach. Doctor Catherine Halsey sometimes got her coffee from a Hawadi Goodwan restaurant, and Captain Jacob Keyes once gave her a diagram of the Keyes Loop which he had drawn on the back of a Havadi Goodwan placemat.
The name is a Bungie inside joke based on how one of the artists would say "have a good one".
An advertisement for Havadi Goodwan.