Why was it made?
The US has long seen a need for weapons that clear a broad area all at once. The Massive Ordnance Air Blast (MOAB) bomb, or "Mother of All Bombs", is a successor to the Vietnam-era 6,800kg "Daisy-Cutter" bomb used to clear trees so helicopters could land. That weapon was also used for mine-clearing in Afghanistan in 2001.
The US developed the BLU-118/B "Bunker Buster" in the early 2000s to penetrate underground targets, and it was used in Baghdad during the US invasion of Iraq in 2003. The same engineer who designed the "Bunker Buster" designed the MOAB.
Why was it used in Afghanistan?
White House press secretary Sean Spicer said the MOAB was dropped to reach "a system of tunnels and caves" used by Islamic State in Iraq and Syria (ISIS) terrorists. The headquarters of US forces in Afghanistan said "the strike was designed to minimise the risk to Afghan and US forces conducting clearing operations" against ISIS fighters and their facilities.
The air force said the weapon is designed to clear targets in caves and canyons, and extensive mine fields, as well as for "psychological effects".
What are its psychological effects?
Military officials have said the explosion of an MOAB feels like a nuclear blast, and a test of the bomb in 2003 produced a mushroom cloud visible from more than 30km away.
Can it be used against nuclear sites in North Korea and Iran?
US President Donald Trump has vowed to stop North Korea's nuclear weapons programme, and has accused Iran of seeking nuclear arms. To effectively strike the nuclear facilities that these two countries have underground, the US would have to use an even larger bomb, the Massive Ordnance Penetrator. It has never been deployed in combat.