It’s common to see old finishes that have bubbled up in areas where they are touched by oily human skin or perspiration. These areas include chair arms and backs and around door and drawer pulls and knobs.
The example shown is the crest rail of an old dining chair. Oily skin and perspiration are slightly acidic, and the acid breaks down the finish over many years.
If you catch the problem early, you can sometimes sand or steel wool through the deteriorated part at the surface of the finish to reveal good finish underneath. You could then apply another coat or two of finish to regain the finish thickness.
But, in most cases, including the one shown, the finish is destroyed all the way to the wood, and the only way to fix the problem is to strip the finish and apply a new one.