Waadi Muhassar is a place between Mina and Muzdalifah. it was here that Allah (swt) destroyed Abraha and his army of elephants. This incident is mentioned in Surah al-Feel in the Quran.
It is sunnah for Hujjaj (pilgrims) to walk briskly pass this area as the Prophet (ﷺ) did because it was a place of punishment from Allah (swt).