Playable versions of the arrangements, always assuming you have a MIDI player. Macs don't have them any more, so you may have to download a player from the app store: I use MIDIplayer X.
They don't sound particularly musical - more like a music-box - as I haven't added dynamics, tempo changes, etc. to simulate a real performance. A lot will depend on your instrument synthesiser and available sound fonts, too: I have specified acoustic classical guitar as the nearest instrument to the uke.
Their purpose is to give an impression of the music, to aid in interpreting the tabs (I find it useful to check that I have read them properly, especially where there is syncopation), and hence to help develop your own performance.