Spotify already has a lyrics feature, so it's not surprising that it's introducing a new related feature. The app adds a Spotify Karaoke feature, which now seems to have a limited release. It is expected to come to users globally in the coming months. It will evaluate you singing your favourite tracks, judging you lyric-by-lyric. It could make for a great party game or just something to do on your own.
It doesn't look like the feature would be limited to Premium users. You can check if you have it by heading into the Lyrics section on your mobile device. There should be a microphone button with the word Sing in the top right corner. Once you open that, you'll see the lyrics and an audio analyzer. It will use your device's microphone to hear/analyze your singing.