That's "The Year of the Cub" in Latin. At the moment, that year looks like this:
The numbers—04, 67, 104—refer to the years since the Cubs' last division, league, and World Series championships.
They had to put another digit on it after the 2008 season. My guess is the current 7 digits will last about 33 more years.
At least they've won a few recently, and have gotten back up to .400. I'm going tonight; we'll see if they can make it to .423.