These graphs are a part of the Energy Performance Certicate - there is more information in the full report including recommendations on energy saving measures. You are entitled to see this report if you are interested in this property. Ask the landlord to show you a copy of the EPC.
The energy efficiency rating is a measure of the overall efficiency of a home. The higher the rating the more energy efficient the home is and the lower the fuel bills will be.
The environmental impact rating is a measure of this home's impact on the environment in terms of Carbon Dioxide (CO2) emissions. The higher the rating the less impact it has on the environment.