Original: In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules (e.g. light hydrocarbons) by the breaking of carbon-carbon bonds in the precursors.
Simplified: In chemistry terms, cracking is a process by which complicated organic molecules (such as kerogens or heavy hydrocarbons) are broken down into simpler molecules (e.g. light hydrocarbons) by breaking their carbon-carbon bonds.