A spreadsheet is a package whereby one can build mathematical models of situations, in a tabular context.

A typical use might be to list the items in a budget for a department in a company, and then to adjust the figures, saving money, to see where benefits occur.

The name is derived from pre-computing days, when this sort of thing would be achieved with large sheets of paper and lots of handwritten calculations, and one would "spread the sheets" over the table one was working on.