The EPC is an assessment of the energy efficiency of your home. Most residential properties in the UK require an EPC when they are marketed for sale. There are a few exceptions, grade 1 and grade 11 listed properties and dwellings under 50m2 may be exempt.
NB: An up to date EPC may just help your property sell quicker. So even if you have one you may want to consider having it re done. Especially if you have had some work done to the property which may effect the rating. If you are unsure whether you have, need or should get an up to date EPC.