Basically it is the number part of a barcode
officially
The EAN system combines barcode symbology and an accompanying number system, to assign each product with an entirely unique reference number
Why do I need a EAN or Barcode?
Pandora, Amazon, CD Baby, Tunecore and iTunes all require that you have a barcode for your Digital Albums. All stores require that you have a EAN ( or UPC in USA /Canada)
The UPC barcode represents the entire digital product. The UPC number can be embedded (but is not required) into the digital album at the mastering stage.
Where do I get one ?
Your digital aggregator will supply one, you may want to get this before completing your final physical artwork so you can supply it to your designer
The digital aggregator will also use this number when they send your music to Itunes etc. alongside the IRSC numbers either provided by you or assigned by them.
Again refer to the FAQ details for your chosen service
So if you intend to sell your cd s in shops the barcode should be included in your Album artwork by your graphic designer. ( on the back)