Barrett's oesophagus (Barrett's esophagus to you Yanks) is metaplasia of the lower end of the oesophagus caused by chronic reflux oesophagitis and is present in about 10% of patients who present with reflux. It is a premalignant condition and is associated with an increased risk of oesophageal cancer.
Current recommendations are for endoscopy and biopsy (looking for dysplastic changes) every 12 months or so whilst the underlying reflux should be controlled with proton pump inhibitor drugs.
Pictures are worth many words:
Eponymous syndromes metanode