A parameter is a property of the legislation that changes over time.
Unlike a variable, a parameter is not specific to a specific entity (person, household…).
For instance:
the amount of the minimum wage;
the amount of family allowance per children;
the marginal tax scale used to calculate the income tax;
the maximum age at which a person is entitled to some benefits;
the list of regions in which a tax is applicable…
Parameters are used in formulas to calculate variable values.