EPC - Energy Performance Certificate is a mandatory, legal requirement when selling your house. It is important to have an EPC as the notary will require the original when the property is being sold...
The actual Energy Performance Certificate gives your property a rating from A (most efficient) to G (least energy efficient). A high rating(A) means that the house will cost less to run for heating etc and is very energy efficient. A rating of (G) will be inefficient, for example, it may lack insulation, have old windows, old energy-intensive appliances etc.
A buyer may use a low rating as some type of bargaining tool as the house will cost more to run than an energy efficient building. Even if your property has a low rating, there is no need to panic...it does not lawfully mean that you have to make improvements, it's just for information.