Site energy is, as it sounds, the energy consumed at the house or building. Source energy is the total energy "at the source". It takes into account all of the energy losses along the way, including power plant inefficiencies, transmission losses, extraction energy (for example, the energy consumed to extract natural gas, oil, or coal out of the ground). On a global, national, political, and sustainability perspective Source Energy is the most relevant performance metric.
For average electricity, the ratio between site and source is roughly 1:4. For each unit of energy that's delivered to the house for on-site consumption, 3 units of energy were consumed along the way.