District F is Founders District. It is the only district with a letter instead of a number. It is called Founders District because the Founder of Toastmasters, Ralph C Smedley, started the first club in Santa Ana, California, within the geographical confines of what is today Founders District.
The first club existed before the first district: District F didn't exist until 1944. Since then, there have been a long list of district leaders, with the top title currently known as District Director. World Headquarters was, as of 2017, located within District F.