This isn't a big move by any means, but it probably spells the end of Eli Whiteside's Cubs career once Welington Castillo comes back. Torrealba has caught over 850 major-league games for the Giants, Rockies, Rangers, Brewers and Padres and caught in the World Series in 2007 and 2011. He's 35 years old, so this is by no means a long-range move, but he surely will be able to hit better than Whiteside, or perhaps even John Baker. He has hit .256/.315/.379 in 3,183 big-league plate appearances with 56 home runs.
As noted in Carrie's tweet, Torrealba has reported to Cubs camp in Mesa, likely to get in shape since he hasn't played since being released by the Angels near the end of spring training this year. I'd expect him to be assigned to Iowa soon, with the possibility that he could replace Baker at some point this year, or be the injury-replacement backup if Castillo can't return soon.