Dharai is plural of dhariah. As a term of Islamic Law, dhariah stands for “a means to a crime”. Sadd (closing) of a dhariah stands for removing the means for it like putting a barrier in between. So sadd al-dharai means prohibiting things/acts that pave the way for certain crimes.
This is a proper and rightful principle of law because Allah the Almighty says “do not approach…”, “stay away from…” while speaking about certain big crimes. Staying away can only be assured by putting a distance in between and this is called sadd al-dharai.
Prohibition of drinking and driving is a modern example of this principle. Neither driving, nor drinking is considered a crime in modern law systems; while drinking and driving is considered a crime in almost every country because of the risk and possibility it has for turning into a more severe crime.