That's the ISO (European) symbol for DC power. It has the voltage listed
next to it, in your case 12V and may also have the max current listed. If it
was AC it would be a "~" over a straight line IIRC. DC is " - - -" over a
straight line "___" These are commonly seem on items which bear a CE mark
(somewhat similar to UL or CSA in North America).
Often the center pin is positive, but by no means always, and the symbol
does not address that.
The other very common ISO sympols you will see is a circle for OFF "O" and a
vertical line "|" for ON e.g. a toggle switch, or a vertical line *in*
circle for ON/OFF e.g. a rotating switch; then there's a horizontal
rectangle with a horiz line thru it for a fuse (with the fuse type, amperage
and min voltage rating printed next to it. There are others, a lightling
bolt with an arrowhead at the bottom for hazardous voltages, and exclamation
point in a triangle for something you should pay attention to or take note