What does this equation mean?

Concorde is the clearest case in transport history of a regulatory filter, not an economic one, capping a technology’s addressable market. The aircraft cruised at Mach 2.02, carried between 92 and 128 passengers, and burned roughly 4,800 US gallons of fuel per hour at cruise, delivering about 15.8 passenger-miles per gallon against 33.3 for the Boeing 707, 46.4 for the 747, and 53.6 for the McDonnell Douglas DC-10 — the direct fuel-efficiency cost of pushing lift-to-drag ratio through the transonic and supersonic regime, exactly the L/D term in the Breguet equation collapsing even as speed M rises [ 11 ] . Its program cost ballooned from an initial 1962 estimate of 70 million pounds to somewhere between 1.5 and 2.1 billion pounds by 1976, with a unit cost of 23 million pounds per aircraft in 1977; against initial predictions of 350 aircraft sold, only 20 were ever built, 14 of them flying commercially for Air France and British Airways [ 11 ] . It was retired in 2003 amid rising maintenance costs, a collapse in passenger numbers after the July 2000 crash, and the broader post-9/11 slump in air travel [ 11 ] .