It adds padding to the left. Very useful for remembering the various string formatting patterns is the following cheat sheet:
Positive values add padding to the left, negative add padding to the right
Sample Generates
String.Format("[{0, 10}]", "Foo"); [∙∙∙∙∙∙∙Foo]
String.Format("[{0, 5}]", "Foo"); [∙∙Foo]
String.Format("[{0, -5}]", "Foo"); [Foo∙∙]
String.Format("[{0, -10}]", "Foo"); [Foo∙∙∙∙∙∙∙]