A lot of data you get are estimates with uncertainty attached. Plus or minus something. Standard error. So when you try to do math with those numbers straight up, ignoring the uncertainty, you end up with a result that seems concrete but it's actually more squishy.
Guesstimate, made by Ozzie Gooen, is an effort to include the uncertainty in your spreadsheets.
The first reaction of many people to uncertain math is to use the same techniques as for certain math. They would either imagine each unknown as an exact mean, or take ‘worst case’ and ‘best case’ scenarios and multiply each one. These two approaches are quite incorrect and produce oversimplified outputs.
Guesstimate works like a regular spreadsheet where you input numbers into cells. But you can also include the uncertainty estimates, which is where it gets interesting. Piece together cells, and then using a Monte Carlo method, Guesstimate generates a new estimate with its own uncertainty.