The license is a modified CC-BY. The only difference from a normal CC-BY is that it specifies that the assets may only be redistributed as part of a game or game-like product. (i.e., no straight-up reselling the assets)
All my assets can be used in any game, whether it's for-profit or not!
The full license text is available to read here: http://untiedgames.com/files/license.txt