What is density altitude ? . I’m Having trouble understanding it

Here is the story of the Shepherd, for those of you who may not have heard of it: The story of "The Shepherd" is a cornerstone of aviation folklore, written by Frederick Forsyth. Set on Christmas Eve in 1957, it captures the transition between the aging piston-engine era and the new age of jet flight. The narrative follows a young, cocky Royal Air Force pilot flying a De Havilland Vampire from West Germany back to England for his holiday leave. What begins as a routine sixty-minute flight across the North Sea quickly turns into a desperate struggle for survival when his aircraft suffers a total electrical failure, leaving him without a radio, navigation instruments, or a way to see through the thick fog closing in on the English coast. As the pilot begins flying "blind" in a triangle pattern, an international distress signal, he resigns himself to a watery grave in the freezing sea. Just as his fuel reaches its limit, a vintage World War II De Havilland Mosquito bomber emerges from the fog. The pilot of the Mosquito, acting as a "Shepherd," uses hand signals to guide the modern jet through the soup of clouds. With pinpoint accuracy, the mysterious escort leads the Vampire to a safe landing at a remote, derelict airfield. The engine of the Vampire flutters and dies the moment the wheels touch the tarmac, saved by seconds of fuel and the precision of the stranger. The emotional core of the story arrives when the pilot seeks out his savior, only to find the airfield nearly abandoned. Upon speaking with a local caretaker and seeing an old photograph on the wall, he realizes that the pilot who guided him home was John "Joe" Kavanagh, a legendary "shepherd" of the skies who specialized in bringing crippled bombers home during the war. The chilling realization is that Kavanagh and his Mosquito had been lost at sea exactly fourteen years earlier, on Christmas Eve, 1943. The story ends with the implication that even in the cold isolation of the cockpit, no pilot is ever truly flying alone.