Primers

Primers, or Primary High Explosives, are super sensitive explosives that do not take much to be initiated.

Typical primer mixes contain lead styphnate, lead (II) azide, tetracene, mercury fulminate, or an antimony-sulfide mixture, and a grit which further sensitizes the mix when crushed by compression. [1] Primary explosives are often a heavy metal attached to an organic compound, which helps the sensitivity as it is ready to react, breaking unstable bonds and forming new ones. Lead azide, for example, with detonate if dropped 6 inches.

Primers are often used to ignite other explosives because they are so sensitive. While they are sensitive, they often to do not have a large output because it takes only a small amount of it to ignite. Therefore, to have an explosive device that can be initiated but has a larger output, you can attach a primary high eThere have also been developments in other primer materials, specifically in lead-free components in order to reduce toxicity of residue and air. 

Sources:

[1] T. L. Davis, The Chemistry of Powder & Explosives, 1943.