expression need to indicate what you want to do, example:
have a table with all sales of your company, the table contains the fields
If you want to know how much each salesperson sold by Seller and Product then Seller and Product will be your two dimensions and your expression will be the sum of the amount of each product sold, in this case:
However if you just want to show the raw data from your table, simply use a box chart table where you need not put any expression simply shows all the fields in the table of your choice.
Sorry for my bad english.