Its an inline if that checks if $measure is set AND its inside of the array. It then timeses $s by the number inside the array.
Its the same as:
$scale = array('minutes' => 60, 'hours' => 3600, 'days' => 86400, 'weeks' => 604800);
if (!empty($scale[$measure])) $s *= $scale[$measure];
if slightly expanding the if makes any more sense to you?