Why do some letters have crowns (Keter)? What is the significance of the crowns on some Hebrew letters?
Every letter in the Hebrew alphabet contains within it certain spiritual powers. The ‘tagin’ (the small lines going up above the letters) represent spiritual powers that are associated with the letter, but are above the letter, rather than in the letter. In the case of the letters with three lines sticking up, the crowns that you refer to, they indicate a great amount of that power being outside of the letter, rather than an integral part of it.