In relation to buildings, energy retrofitting is anything done to improve the energy efficiency of an existing building. This usually includes upgrading the roof and wall insulation to help keep the heat in, and installing renewable energy systems like heat pumps.
Examples: improving wall and roof insulation, and upgrading the heating and hot water system to systems using renewable energy like solar panels and heat pumps.
You can find more information about this on the Sustainable Energy Authority of Ireland (SEAI) website at www.seai.ie