AUX is an external input port, which is equivalent to using the speaker as an external speaker. You can connect one end of a cable with earplugs at both ends to MP3 or other playing media, and insert the other end into AUX port to play with MP3.
The SD card slot is for you to insert SD (nonsense). Choose to play the SD card after insertion, and the system will automatically identify the music files in the SD card.
Similarly, USB is a port for connecting other storage media, such as a mobile hard disk. Plug in the power and you can play songs on the mobile hard disk.