Romberg is a simple procedural test for ataxia (lack of balance). The cerebellum is the lowest and back most part of the brain and largely responsible for movement, coordination and balance. A lesion, in this sense, would be any damage or defect to the region.
Romberg test is used to assess possible problems related to the cerebellum.
Here is a brief description:
Neuroexam.com - Romberg Test