Calculated field - as it sounds a field that is calculated. For example, if we have a measure Sales and a measure Quantity we can create a calculated field like:
[Sales] / [Quantity] and then call that new field Sales per Quantity.
My suggestion is that you go in to Tableau, click Create Calculated field and then have a look at the different calculation types:
Parameters - is a value that you can use in for example calculated fields. For example, if you would like the user to choose wich measure to use for your bar graph then we can create a parameter like:
Then you can create a calculated field like:
IF [Parameter 3] = '1' THEN [Sales] ELSE [Quantity] END
Hope this helps.