First, the Twins outrighted Starting Pitchers Dave Gassner and Justin Jones and Infielder Terry Tiffee from the 40-man roster. With 6 years in the minor leagues, Terry Tiffee becomes a minor league free agent and can sign anywhere. Gassner and Jones are still the property of the Twins, but are now subject to the same rules as any other minor league player not on the 40-man roster, including being subject to the Rule 5 Draft.
Tiffee makes the most sense; he was out of options and so would have had to either stay with the Major League team next year or clear waivers to get sent to the minor leagues. It was pretty clear that Tiffee wasn't going to fill a role in the Twins system, so clearing the roster space now instead of waiting until the spring makes a lot of sense.
I'm a little more curious about the decisions on Gassner and, especially, Jones. Gassner was injured most of the year, but he had an option year left and is a left-handed starter; in other words, he's the kind of guy you usually give every opportunity to succeed. He was made expendable by the emergence of guys like Boof Bonser and Matt Garza. Jones is the pitcher the Twins got from the Cubs in the three-way Doug Mientkiewicz deal a few years ago. Sadly, he regressed this year, being demoted from AA New Britain to High A Ft. Myers relatively early, and struggling there with an ERA of 5.20. Jones departure means that the Twins essentially got nothing but roster space in the Mientkiewicz trade.
The Twins also "activated" Shannon Stewart from the 60-Day DL, a move that will be meaningless as soon as the free agency period opens and Stewart files. Nevertheless, the move means that the Twins 40-man roster currently stands at 38 players.