A sound-interface, also known as sound-card, audio-card or audio- interface is hardware that provides audio inputs and outputs for your computer, tablet or smart-phone. In all these devices you already have internal sound card, but you cannot connect an extra microphone.
To extend the number of inputs or outputs, or to be able to have a better quality sound overall, you can use an external sound card.
If you need a basic, affordable sound card to use for simple audio/video recording of yourself playing an instrument or singing check Behringer U-Phoria UM2 or Behringer U-Phoria UMC22. These will give you the option to connect a better quality microphone and/or instrument (Guitar/Bass) to your computer. This improves the audio quality of your recording since you can place this microphone closer to your instrument and in general these microphones are more focused (directional) compared to the less focused (omni) microphones built in the computer or tablet/smart-phone.
Also you will be able to listen to yourself better on headphones (using the headphone output of the sound card) or speakers (using the stereo outputs of the sound cards). The UM2 sound card has RCA (cinch) outputs which you can connect to an ordinary HiFi amplifier, while the UMC22 has jack outputs and can connect to studio monitors which have built in amplifiers.
Some sound cards require drivers in order for the computer to be able to detect them (especially when used with Windows). Check the manual or on-line videos for more information.
Check out the video bellow for further information about the UM2 sound card.