An audio interface is the bridge between microphones, instruments, and your computer. It converts analog sound waves into digital data (A/D conversion) for recording in a DAW, and converts digital audio back to analog (D/A conversion) for playback through headphones or monitors.
Key specs include: number of inputs/outputs, preamp quality, sample rate support, and latency (the delay between input and output). Popular interfaces include the Focusrite Scarlett series, Universal Audio Apollo, and Audient iD.
For home studios — increasingly common in West African production — a good 2-input interface is the single most important hardware investment.