Execution plans show the steps SQL Server takes to execute your query. Each icon in the graphical execution plan is known as an operator, and the most common way to read a plan is by starting with the top right most operator and following the arrows to the left.
When you reach a join or concatenation operator where multiple branches merge into one operator, you can proceed to the right-most operator of one of the lower branches and start the process of reading right to left again. In general, this can be summed up as reading a plan right to left, top to bottom.
Right-to-left, top-to-bottom gives you the flow of information and that’s quite important. To understand how the engine works, though, you also need to read left-to-right, as Brad Schulz’s outstanding one-act play demonstrates.