The object of the game is for the player to form hexagons of a solid color (called a "Hex") by rotating into place and combining together colored triangles which drop down from the top of the screen. The player is required to create a certain number of Hexes in order to pass each stage. Initially the game starts with triangles encompassing only two colors, with more colors eventually being added to the mix. In addition, glowing triangles give the player power-ups such as creating a hole in the bottom of the play area, or swapping colors. There are eight different colors in total.
Originally released as Dialhex as part of the Japanese-only Bit Generations series, it was later re-released as "Rotohex" on DSiWare.
Source: Wikipedia, "Rotohex", available under the CC-BY-SA License.