Part of the story is the word; Threshold. The story tells that the meaning is that it was common to spread “thresh” (presumably reeds or rushes) on the floor of one’s house to prevent slipping, necessitating the addition of a piece of wood in the bottom of the doorway, called a “threshold,” to keep the thresh from “slipping outside.” Voila, our modern word “threshold” for the bar of stone or wood at the base of a doorway.
Threshold is a very old word, dating to circa 1000 and probably earlier. But threshold has nothing to do with “threshes” on the floor. The word threshold first appeared in Old English as “therscold” or “threscold.” The beginning of the word carried the meaning of “to stamp with the feet, to stomp noisily,” which is, of course, what one does when entering a room with mud or snow on one’s shoes. The second part of the word is a mystery, but it is fairly certain that it was something other than our modern word “hold,” and it was transformed into the more familiar “hold” over time.
The threshold is literally the first place in a building you step and has evolved to mean any gateway or entry point.
