In the specific snippet you provided, the CSS defines how a particular element behaves on the screen:

Because these class names are generated by compilers (like Closure Compiler), they don't have "semantic" names (like .header-top ). Instead, they serve as unique identifiers for specific styling rules that can change every time the site's code is redeployed. The Breakdown of Your Snippet

: Short names like y47QBqv2 take up much less space than main-navigation-search-button-active . On a site with billions of users, saving a few bytes per page load reduces massive amounts of bandwidth.