What is an audio interface?
an audio interface takes all your inputs and outputs and dose the conversation processing so the data can be read by the audio driver/software
How it processes the Audio
The audio inputted by XLR or Line in jack makes its why into the Mic Pre where the audio gets boosted by multiplying the voltage making the audio signal louder. the analogue signal that the cable provided is converted to digital for the DSP to allow your computers drivers to provide the software with the inputted audio. Then the audio gets sent back to the DSP so it can get converted from digital to analogue at the DAC allowing it to be outputted at Headphones or an Amp where it can be played out of speakers.