is the number of possible configurations of a quiescent10-by-20 Tetris board. It's the evaluation of the function (really, expression: the function itself would have 20 as a variable):

    \       /         \  n
     \     |   10      |
1 +  /     |  2   - 2  |
    /       \         /

     n = 1

The explanation for this: each possible Tetris row has one of 2^10 (1024) possible configurations, as each space may be in the state of a block "present" or "not present". There is one possible configuration which does not have any rows, the completely empty board, which accounts for the 1 in the equation.

The number of possible boards with exactly one non-empty row is 1022 (2^10 - 2), as no completely empty rows are allowed below any non-empty rows, and any full rows will be cleared. As for the number of boards with exactly two non-empty rows, it will be (2^10 - 2)^2, and so on from 1 through 20. Using sigma notation, the number is the summation which evaluates to the aforementioned huge number.