The model is a simulation of the discharge which might occur when a human touches an electronic device.
The HBM definition most widely used is the test model defined in the United States military standard, MIL-STD-883, Method 3015.9, Electrostatic Discharge Sensitivity Classification.
This method establishes a simplified equivalent electrical circuit and the necessary test procedures required to model an HBM ESD event.
A similar standard, IEC 61000-4-2, is used for system level testing and quantifies protection levels for a real world ESD event in an uncontrolled environment.
In both JS-001-2012 and MIL-STD-883H the charged human body is modeled by a 100 pF capacitor and a 1500 ohm discharging resistance.