Barn Alarms are not for owners per se. The only way you can hear them is if you have the Barn Alarm window open (which I usually don't). It alerts you to any and ALL alarms going off, which on some nights can be quite busy. Moderators and Cam Owners can activate any alarm at any time. I believe registered members can activate an alarm after 3 individuals click it. Its mainly so others can be aware that a mare is foaling and watch. The best thing to do to alert a cam owner is to call them off the phone numbers posted at the top of the cam page.
High alert is for water breaking or a visible foot/sac. Low alert is for mares acting foalish ie getting up and down, etc.