A field parameter lets a reader switch the column or measure used by a visual. You define the allowed fields, add the parameter to a field well, and use its slicer to choose an entry. The report can show sales by Region, Category or Segment without maintaining three separate charts.
The parameter changes the query shape. A normal Region slicer only removes rows; it can't replace the Region axis with Category. Choose sensible names and start with one selected entry so the chart's meaning is clear. Keep the parameter disconnected from the business tables. Measures must still respond to ordinary slicers, and a field choice should not reset unrelated page filters. Test a different entry and return to the default before sharing.