What does ‘&.’ in ‘&.sub-title’ indicates in scss?

The & concatenates the parent class, resulting in .title.sub-title (rather than .title .sub-title if the & is omitted).

The result is that with the & it matches an element with both title and sub-title classes:

<div class="title sub-title"></div> <!-- << match -->

whilst without the & it would match a descendent with class sub-title of an element with class title:

<div class="title">
  ...
    <div class="sub-title"></div> <!-- << match -->
  ...
</div>