A barium swallow is a test which is done to look at the oesophagus, not for function but for anatomy , looking to see if there are any anatomical/structural problems such as webs, pouches, strictures/narrowing to name a few.
It is very helpful in children who are having swallowing difficulties. It is also helpful in looking for hiatus hernia but doesn’t tell us anything really about any inflammatory problem in the oesophagus.