Sometimes it is very useful to be able to see white space characters in source code files. For example, it is a clean practice to avoid having trailing spaces after code lines, and even more important to keep your indentation consistent: using all tabs or all spaces across the whole file.
To spot those problems you can display these characters in the editor by adding the following lines to your vemrc file:
set listchars=tab:»\ ,trail:· set list
With listchars, you can specify which characters to use to display both tabs and trailing spaces. For spaces you can provide only one, in this example ·. For tabs, you can provide two. If your tab is, for example, 4 characters wide in size, the first one (») will be placed at the beginning and the second one (an empty space escaped with backslash in this case) will be repeated for the remaining three characters.
The list option enables the display of white space characters, and nolist (which is the default) hides them.
There are more display options and more cases that can be shown apart from these two. Check the listchars option in the help documentation to get more information: