Tiny Player for Mac is a minimalistic application that allows you to create a playlist from .mp3 files and a few other formats (FLAC, AAC, AIFF and WAV) and then play the files. Before you quit the application you can save the playlist for later use. Popular formats like .mp4 and .flv files are not supported, which means, music videos (mp4) and many YouTube music downloads (flv) cannot be played.
People who "highly recommend" this software here or say it's "Exceptionally useful"(Fariborz Dec 18 2018) must be people who just want to listen to music and are happy with being unable to do anything else: no sorting, no tagging, no commenting, happy not being able to create smart lists, happy not being able to search for tracks in the list, and not having any place for lyrics. The less the software can do, the happier that seems to make those people.
As an alternative free player for more ambitious music lovers I recommend
VLC Media Player. It can play .flv, mp4 and most other music and video formats you throw at it. <https://www.videolan.org>