An 'audio interface' is basically any computer gadgetry that converts real sound into digital - and back out again. Computers generally have them built in (e.g. the sound card) but for sound recording and production there's more to it. Conversion needs to be fast for clarity and processing those masses of sonic data. If you are recording a voice, for example, you won't want 'latency' - that's delay between singing and then hearing yourself, and a quality interface has that covered.
Interfaces can also have a massive range of features: multiple inputs (e.g. guitar and vocal at the same time, or a whole band), pre-amps to improve the tone of anything you want to plug in, software, effects, even mixing capabilities.
Ultimately, you'll need to ensure that your new interface works with your setup, the way you make music and gives you the end result you are looking for.
So feel free to browse and buy on-line with us and if you would like to explore and ask questions then come and say "hi" to us at a Mega Music store.