CLEAR THE LINES! SHIFT AND ROTATE THE BLOCKS TO FIT TOGETHER INTO A SINGLE LINE. THE LINE WILL THEN DISAPPEAR, AND YOU CAN CONTINUE BUILDING. DON'T LET THE BLOCKS REACH THE TOP, OR IT'S OVER!
when in multiplayer, your opponents will also be clearing lines. these lines will be sent to you as "garbage," or the gray blocks in the above image. they make your own colored blocks reach the top faster, which is not good. COMPLETE THE LINES OF GARBAGE TO STAY AFLOAT!