That's the spoke hole eyelet, pulled loose from the rim. Depending on the rim construction, it may be possible to re-peen it back into position. Remove the tire, rim tape/strip and spoke, support one side of the eyelet on something solid and use a blunt punch to re-peen it into place, like setting a rivet. This works best on single-wall rims; double wall rims have different eyelets and failure like yours would be difficult to fix. Also, check around the spoke hole for cracks or other damage. If there are cracks, the rim is failing and should be replaced. If it looks intact, but you can't get the eyelet re-set in the rim, you may be able to use a nipple washer on the inside to distribute the stress from the tensioned spoke and protect the rim. That was standard practice BITD before most rims had eyelets.