An audio interface is a link between a microphone (or other sound sources) and a computer. It is a kind of bridge between analog and digital signal. As a rule, the audio interface is equipped with a microphone preamplifier, line-in, and many other connectors.
You may wonder if there is a preamp in the audio interface, isn’t it easier to buy a conventional EQ/Compressor or preamp? The answer to this question is explained by the need for analog-to-digital conversion. Standard preamps and equalizers/compressors transmit an analog signal, and a digital signal is needed to process audio on a computer. The audio interface converts the analog signal to digital, which is then transmitted to the computer.
At https://musiety.com/the-best-audio-interfaces/ you can learn how to select the best audio interface.
If you are seriously engaged in recording, then soon the limited capabilities of the computer sound card will no longer satisfy you. In most cases, ordinary sound cards are not equipped with high-quality converters. These cards are quite suitable for playing games and listening to MP3-songs, but they lack the headroom, as well as the power to preserve the integrity of the signal.