When a Multimino lands, the piece separates its blocks by their color. If the color of blocks do not match, the blocks separate and fall down if there is space below. When you drop a Tetramino or Multimino, adjacent blocks of the same color will combine. This combined piece is called a Polymino. If a Polymino becomes too big, it is more difficult to Cascade.
Cascades are a special feature of The Next Tetris that make it unique.
If you completely fill in one or more horizontal lines, those lines of blocks will disappear. The blocks above those cleared lines will fall individually, possibly completing their own lines in turn. This phenomenon is called a Cascade. As you get better at playing The Next Tetris, you can even clear more than 5 lines at once by using Cascades. The space in which pieces fall is called the playfield.