Thorp was a referee for the NFL back in the 20s. He died young, but was liked and respected by the owners and players around the league. Everyone thought of him as the best official, and fair with everyone.
He died a short time before a new season started, and the owners had decided on a trophy for the winners. There would be a traveling trophy which would be engraved with every year's winner, and it would be passed around. There would be a second trophy that was the same, but a lot smaller, that each team would keep.This was the ultimate trophy before there was a Super Bowl trophy.
Since Green Bay won it more than anyone else, when it was discontinued, because there was a Super Bowl, and trophy, the Packers got to keep the original traveling Thorp because they were the last ones to win it. It was similar in concept to the Calder Trophy which goes from winning team to winning team in the NHL.
Interestingly enough, there are two historical Championship trophies that no longer are in existence. The Thorp, and the Super Bowl Trophy. The Packers own both of them. The Super Bowl trophy was named after Lombardi before Super Bowl III when the Jets won it. The original no longer existed.
Just another important part of history that adds to the Packer legend.
Added Note: I forgot to mention that the engravers erroneously put the name Thorpe on it, in their mind, thinking of Jim Thorpe. A different cat. They didn't do spell checks in those days.