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.

To answer your comment to Puttafish,

Why do they draw it on the power supply? Why not near the connector?

The connectors are purchased with no polarity assignment. It's up to the PSU designer what the polarity is or if it is AC. Thus it makes sense for the PSU builder to label the polarity on the PSU along with all the other technical information.

There's not a lot of room on the connectors either!