The DragonFly Red is a USB DAC (digital-to-analogue converter), preamplifier and headphone amplifier. It bypasses the audio circuitry in laptops, computers and smartphones to deliver cleaner, clearer, more naturally beautiful music to headphones, powered speakers or complete audio systems.
The 2.1 volt output makes it more suitable for driving demanding headphones compared to the 1.2 volt output of the cheaper DragonFly Black.
I have used the DragonFly Red in my iMac to feed a pair of powered Bose desktop speakers and there is a noticeable improvement in sound quality compared to using the 3.5 mm audio output of the iMac. I also used it to connect my iPhone to my Beyerdynamic Amiron wired headphones (using a USB to USB-C adaptor) and found it to be quite capable of driving these demanding headphones, which have a 250 ohm impedance.
As I am now selling my Bose desktop speakers and Beyerdynamic headphones (just bought some Apple AirPods Pro, which are more convenient for travelling!) I no longer have a need for a DAC, hence my reason for selling.
The DragonFly Red is still in excellent condition and complete with storage pouch, instruction manual and original box.