The Threshold operator uses the threshold variable to determine whether cells remain in the visualization. The threshold variable is usually the plotted variable in which case the Variable variable button displays: default. To specify a threshold variable other than the plotted variable, click on the Variable variable button and select a new scalar variable from the list of available variables.
You might set the threshold variable when you apply the Threshold operator to plots which do not take scalar variables as input. An example of this is the Mesh plot. When you apply the Threshold operator to a Mesh plot, you must set the threshold variable to a valid scalar variable for cells to be removed from the plot. You can also use the threshold variable to remove cells based on one variable while viewing the plotted variable.