Turning on Marks adds visual guides to the view, making it easier to see whitespace. The Marks menu is under View | Marks. You can also change the defaults for the configuration by going to Tools | Properties for Current Configuration and open the Marks page.
There are multiple types of Marks. You can enable one of them or have multiple enabled at the same time.
This shows where the newline characters are in your document. If you enable CR and LF with Different Marks, it makes it easier to differentiate between CR, LF, and CRLF line endings.
End of File
This adds a mark after the last character in the file.
This indicates an ideographic space.
These show as red vertical lines to help with indentation.
You can set a vertical guide at the cursor position with Add/Remove User-Defined Guide in the Marks menu or go to Tools | Properties for Current Configuration and open the Marks page to input User-Defined Positions.