It converts the USB signal of your streamer to AES/EBU, I2S or SPDIF for the DAC.
The HE version has a built in high end power regenerator and its a superb unit for about 1k.
I have the non HE version (about 500 bucks plus shipping) and it is considerably better than my Gustard U16.
I havent heard the Singxer SU-6, but some folks that have them both say that the DI-20 and the DI-20HE are considerably better.
Look for it on head-fi forum. There are pages about it.
Its also FPGA programable and it gives you the chance to make it sound better with new firmwares that are launched every few months.
Audio-gd is a hidden gem in the audiophile world. Incredible performance for the price.