Fields are the columns in the data (and can be calculated fields too). A field should always be surrounded by square brackets, [], if they contain spaces or are not unique in name. It is best practice to surround fields with []. Fields are shown in orange in Tableau. You can drag and drop the field directly into the calculation from the Dimension or Measures tabs. Alternatively, you can start typing the field name and Tableau will show you options to autocomplete fields, as shown in the following screenshot: