This is an Apple AirTag, which is a tracking device designed to help locate personal items.
AirTags use Apple's Find My network to help you locate items like keys, bags, or other personal belongings.
They require an iPhone or iPod touch running iOS 14.5 or later, or an iPad running iPadOS 14.5 or later.
AirTags have features like Lost Mode, which notifies you when your AirTag is detected by a device in the Find My network, and the ability to share contact info via NFC tap.