What does this plus (cross) sign on this electrical panel mean?
(airliners.net)

Above I marked five similar plus signs on the 737 (there are more). Beneath the +'s are the contacts for the backlighting of each panel. If it acts up then pressing on the sign will ground/secure the connection and may solve the issue.

The electroluminescent light-panels are flexible and the ride can be too bumpy sometimes. Better push the right spot than wait for a specialized someone on the ground who knows where each contact is.

Composite based on images from b737.org.uk.