It is this principle on which all electrostatics is based. In general, a material is either a conductor or an insulator. Each type of material has a different arrangement of atoms, electrons, and protons. The particular arrangement of a material's atoms can be very advantageous for electric charge to travel through it. Metals, for instance, are very good conductors.
Charges can flow through a metal with very little resistance.
The particular arrangement of a material's atoms can by disadvantageous, however. Rubber, for instance, has its atoms arranged such that it is very difficult for electric charge to flow through it. Rubber is considered to be an insulator. A person's body acts as a conductor. Surprisingly, a person's body allows electric charge to travel through it easily.