A parameter is a value like string, integer, float, date and it can be used to replace the constant value during creating a filter, calculating field and reference line etc. for example if you want to analyze the records where sales is greater than 50000$ and where the sales is less than 50000$, for that you have to create a calculated field that will returns TRUE when sales>50000$ and FALSE when sales<50000$. So in this calculation you can replace the constant value with a parameter and use the parameter control to vary that threshold. So in this way you can dynamically change the threshold value in your calculation. If you use Parameter Actions then you can make the parameters more dynamic and interactive because it will allow the user to change the parameter value by just clicking or selecting the mark.
To learn more about parameters in tableau check learn tableau or get indepth knowledge about this field through Tableau Certification Course by Intellipaat.