DESCALE! - MEDIUM CUP - Error Code - Built-In Coffee Maker

It indicates that descaling is needed.

• Depending on the mineral content of your water, calcium deposits or scales may build up in your brewer. Scale is non-toxic but, if left unattended, can hinder coffee system performance. 
• Calcium deposits may build up faster, making it necessary to descale more often. 
• The descaling program must be run as soon as possible.

NOTE: Failure to descale the machine regularly can result in decreased product performance and may result in product damage. If the descaling cycle is interrupted before it has been completed, it will need to be restarted from the beginning.

Check out Descaling the Coffee Maker for additional information.