What is exactly is barrett's esophagus?

Metaplastic change. Dr edmison is correct. The clinical importance is that this metaplastic change increasing one's risk for esophageal carcinoma.
Barrett's. This is a change in the esophagus in response to longstanding reflux. It is a precancerous change which requires frequent surveillance EGDs to make sure cancer is not developing.
Precancerous entity. Barrett's esophagus occurs when metaplastic columnar epithelium (that predisposes to cancer development) replaces the normal tissue of the esophagus.