This thread is for the Library tab.
What it's for: playing the music and videos already on your computer — MP3, WAV, OGG, FLAC, M4A, Opus, AAC and more — by album, artist, genre, song, folder or video, as a list, a grid of covers or a Cover Flow.
How it works: everything appears the moment you add a folder, and the tags, covers and lengths fill in behind it, so a big library is usable while it is still being read. Covers come from the files themselves or a cover.jpg beside them — nothing is fetched from the internet. Playback carries on while you use any other tab, with Up next and playlists down the side, and songs can remember the sound you tuned them with.
Ask here about adding folders, tags and covers, playlists and Up next.