"I'll Be Waving As You Drive Away" is the moment where Mary Ingalls (Melissa Sue Anderson) goes blind.
Between the show and Mary's actual life, Mary had wanted to be a teacher.
Real-life Mary never did become a teacher, and lived with her parents until their deaths, then with her two younger sisters, Carrie Ingalls-Swanzey and Grace Ingalls-Dow, until her death.
But in the show, Mary went off to Vinton, Iowa in real life (but the show had Mary go off to Burton, Iowa to learn how to function after losing her sight, where she met Adam Kendall (Linwood Boomer), who was her teacher, and later, husband, when Adam invited Mary to teach at the blind school in the fictional town of Winoka, Dakota Territory (based on the Ingalls' final move to De Smet, South Dakota).
While watching the episode, I wonder if the show was trying to show that, despite Mary's blindness, she could fulfill her dream of wanting to be a teacher.
In the show, Mary went on to teach at the Winoka School for the Blind alongside Adam Kendall, who would (at least in the TV show) become Mary's spouse, later moving the school back to Walnut Grove until a fire destroyed the blind school, which would eventually move the blind school to Sleepy Eye, Minnesota.