This is a pentacube placement game. At the start of the game, the players choose twelve pieces apiece. They then take turns placing the pieces squarely into the box so that nothing protrudes above the top edge.
Players try to form "reserves," which are holes in the structure that may only be filled by pieces that they themselves hold.
A player who passes is eliminated and loses. Every piece left in the loser's hand counts as a point for the winner, but the other player may continue placing pieces in order to rack up extra points, one per piece so placed.
There are minor variants of this game in the rule booklet for three and four players, as well as a quick game called "Home and Dry" for two through 12(!) players. There was apparently also a separately obtainable second booklet, entitled "Spiele mit Pentakuben," that provided eight additional games and a variety of puzzles that could be built with the pentacubes.