Sass supports both invisible and visible comments. Using // before any Sass, this will place a comment in your code, but will not be output in the processed CSS. Using the standard /* */ CSS comments in your Sass, when processed this will be output in your CSS.
Leaving comments or instructions in your code is just good practice. I find it essential to leave good instructions behind about my code using the invisible technique as I begin to engineer increasingly more complicated Sass.
The following example is a sample of code from the Compass library illustrating a good use of comments.
// override to change the default
$default-background-size: 100% auto !default;
// Set the size of background images using px,
// width and height, or percentages.
// Currently supported in: Opera, Gecko, Webkit.
//
// * percentages are relative to the background-origin
// (default = padding-box)
// * mixin defaults to: `$default-background-size`
@mixin background-size(
$size-1: $default-background-size,
$size-2: false,
$size-3: false,
$size-4: false,
...