Validation enables you to restrict the values that a User is allowed to
enter into a cell - you might have a formula which expects integer
values, for example, so you don't want to allow any fractional values.
Or, you might calculate the square root of a number - obviously, you do
not want negative numbers to be entered. You can also restrict input to
only certain values, by setting up a list in the sheet and only
allowing choices to be made from that list (via a pull-down).

To apply validation, select the cell(s) you want this to apply to and
Data | Validation - the dialogue box allows you to specify the
restrictions and also on other tabs you can specify an optional message
which you might want to display when the cell is selected (e.g. "Please
enter positive integer") as well as the error message to display and
the action to be taken when the data is not valid.

Hope this helps.

Pete