Ammonium nitrate is used to make fertilizer and bombs. The US has many “stashes” of ammonium nitrate around the country (very common stuff). However, due what it can do, it must be stored carefully. In its solid form, ammonium nitrate relatively stable. However, ammonium nitrate decomposes rapidly, releasing nitrous oxide - a toxic and VERY flammable gas. ANY ignition source will ignite this. Once ignited – kaboom! Ammonium nitrate is also an oxidizer, meaning it increases the burning of fuels by increasing the available oxygen. In other words, the more ammonium nitrate, the bigger the kaboom.
According to Hassan Diab, the Lebanese Prime Minister, over 2,900 tons of ammonium nitrate had been stored at that site since 2014.