DC barrel plug with 5.5 mm outer diameter and 2.1 or 2.5 mm inner diameter are common in electronics world. While you can certainly insert a 2.5 mm inner diameter plug (male) to a 2.1 mm jack (female), the other way round might not be true.

This adapter converts a 2.1 mm inner diameter barrel plug to a 2.5 mm inner diameter. If your device can only support 2.5 mm plugs, this adapter is useful.

Take note that this adapter does not perform any voltage conversion. Please make sure that your DC plug meet you device power requirement.