What does an underscore “_” mean in CSS?

It’s one of a number of CSS “hacks” used to target specific versions of Internet Explorer.

selector {
    property: value;   /* all browsers */
    property: value\9; /* < IE9 */
    *property: value;  /* < IE8 */
    _property: value;  /* < IE7 */
}

Generally speaking you should avoid CSS hacks in favor of conditional classes on HTML.