What does this symbol on the box of power supply mean?
Negative on the outside of the barrel connector, positive on the inside of the barrel connector.

It is the standard method of showing the polarisation of the barrel connector. The positive pin is usually (as in this case) the inner contact, and negative is usually the outside of the barrel.

If you look at the image, it shows a surrounding area labelled "-", an inner point, which it labels with a "+". Hence, negative surround, positive inner contact.