Complex sentence: An aromatic (or heterocyclic) amine quickly reacts with a nitrite to form an aryl diazonium salt, which decomposes in the presence of copper (I) salts, such as copper (I) chloride, to form the desired aryl halide.
Split into simpler sentences: An aromatic (or heterocyclic) amine will quickly react with a nitrite to form an aryl diazonium salt. This diazonium salt will decompose in the presence of copper (I) salts (such as copper (I) chloride) to form the desired aryl halide.