What is this battery trick I have seen but no real details about it?

Thank you for the tips

So got my first juke and was free. Didn't test until I got home. Just put it in the truck and brought it home. So I put in some CDs and fired it up. It will pick up the disk, spin it, then screen says cd off cd on. I do notice that the battery light is on and I did change them with the machine on. A...