The "Sign of La Forge" was a premonition invented by Lieutenant Worf in 2370, named for Lieutenant Commander Geordi La Forge. It consisted of a set of crisscrossing horizontal and vertical yellow lines on a black background, appearing as a faint image in water. Worf claimed it appeared to travelers as a sign of good luck and fortune.
In reality, Worf was masquerading as a Boraalan seer while the group was being moved from Boraal II to Vacca VI on the USS Enterprise-D holodeck. The hologrid became visible to a Boraalan, and Worf improvised the term to explain the malfunction and alert La Forge, who had been monitoring the situation over a communications channel, to the problem. (TNG: "Homeward")