Backslash in PHP -- what does it mean?

It's because they're using PHP namespaces. Namespaces are new as of PHP 5.3.

It's PHP's namespace operator: http://php.net/manual/en/language.namespaces.php.

Don't ask why it's a backslash. It's (imho) the stupidest possible choice they could have made, basing their decisions on a highly slanted/bigoted scoring system that made sense only to the devs.

This is syntax for namespaces. You can read more about namespaces at PHP documentation. They they require at least PHP 5.3.

For example:

namespace SendGrid;
function Mail() {
    // You can access this function by using SendGrid\Mail() externally
}