\
has no specific CSS meaning aside of escaping the next character. Here it allows to include the :
as part of the group-hover:block
class name and avoids interpreting it as a pseudo-selector (all of which are starting with the :
character).
Regarding the hover control, it is actually implemented through the inline styles of your linked example. For instance <a class="font-semibold whitespace-no-wrap text-blue-600 hover:text-blue-800"
means "apply the font-semibold
, whitespace-no-wrap
and text-blue-600
CSS classes on this anchor, and apply the text-blue-800
class on it when it is hovered".