Estimated Execution Plan: meaning of "Output List"

We have a query that that includes a SELECT Distinct.... clause. The estimated execution plan displays an icon with text below:

>>>>>>>>>

Sort

(Distinct Sort)

73%

>>>>>>>>>

I guess this means the "Sort Distinct" takes 73% of the query cost. When I hover over the icon a yellow box appears showing:

Estimated number of rows: 3854150000  (no joking!) and an "Output"  list containing some recognizable table\column names and extra items such as  Expr1034, and Expr1035. What does all this mean? 

TIA,

edm2