When a bomb exploded in New York City in 2016, the explosive component was later accursed of being Tannerite, something that had been largely unheard of outside of the world of target shooting. In target shooting, Tannerite is used for small explosive targets so that shooters can practice their aim and get feedback when they hit what they're aiming at.
Tannerite is a brand-name explosive. As a binary explosive, it's a two-part mix that must be combined in order to be active. For target shooters, they purchase the two components separately and then mix them together when they're ready to use it. Tannerite's formula has been adapted slightly through the years, however, Steve Yerger says the current proprietary formulation uses "just ammonium nitrate and powdered aluminum."
A generic mixture of these two substances is properly called "ammonal", but just like we call any facial tissue a "Kleenex", the generic mixture used in the explosive is being called by the brand-name Tannerite.
Since neither component is explosive on its own, the federal government really doesn't regulate the substances. The reason why it was used is that it's easy to get ahold of. Because binary explosives require two components to be mixed, they are considered relatively safe. However, when mixed and exploded in a container, they are considered dangerous because, although the explosion is small, the container can cause shrapnel to fly and hit someone.
Even once mixed, the two components need a burst of energy to set off the reaction. In target shooting, this is a bullet being fired. Whether or not Tannerite was actually used in the New York bombing remains an active investigation. Without a high velocity bullet to detonate it, both Yerger and law enforcement have struggled to determine whether or not Tannerite, or some generic form of it, was actually used.