A parameter is a single value the reader can change: a number, a text or a date. On its own it does nothing; it works when something uses it.
Create one with Analysis > Create Parameter...: give it a name, a data type, a current value and the allowable values (All, a List or a Range). It appears under Parameters at the bottom of the Data pane. Right-click it and choose Show Parameter to put a control on the sheet: a slider for a range, a drop-down for a list, a box for All.
Three common uses:
- Top N: on the Top tab of a filter, choose the parameter instead of a fixed number.
- Thresholds:
[Discount] >= [Discount Threshold] in a calculated field.
- Swapping measures:
CASE [Show Measure] WHEN "Sales" THEN SUM([Sales]) ELSE SUM([Profit]) END.
A parameter is one value for the whole workbook, unlike a filter, which keeps or removes rows.