(colloquial) Used before a plural noun to form a compound noun having the sense of: the greatest or largest of its kind.
2003, "2003 Movie Guide", Christian Science Monitor, 26 Dec 03:
Driving to a dinner engagement, a Parisian woman gets stuck in the mother of all traffic jams, offers a ride to a handsome pedestrian, and enters a fleeting affair that catches both of them by surprise.
2006, Jean Chatzky, "Get the Scoop", Money, vol. 35.8:
Five mail-order ice creams. Four pregnant women. Welcome to the mother of all taste tests.
Appendix:American Dialect Society words of the year
Etymology
Calque from Arabic; popularized after its use by Saddam Hussein, then president of Iraq, in reference to the Gulf War as ام المعارك(umm al-ma‘ārik, “mother of battles”).