Check out example codes for "sass set variable if not defined". It will help you in understanding the concepts better.

Code Example 1

$content: "First content";
$content: "Second content?" !default;
$new_content: "First time reference" !default;

#main {
  content: $content;
  new-content: $new_content;
}

/* 
 * Compiles to
 * #main {
 *   content: "First content";
 *   new-content: "First time reference"; }
 */

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