That's the advice for any active shooter situation. You run, and get as far away as fast as you can. If you can't run (i.e. the exit is either covered by a gunman or otherwise can't be reached safely) you hide. You hide as quietly as possible in as hard to access of an area as you can create. Lights off, doors closed, locked, barricaded, and secured. If you can't hide (i.e. there's nowhere to hide or nowhere safe to hide) you arm yourself with anything heavy, blunt, sharp, etc. you can find, whether it's a kitchen knife or a baseball bat or a wooden panel, and you prepare to fight. If you're lucky, you can do it by hiding next to a doorway and striking if the overconfident gunman strolls in. You fight to kill. You don't stop attacking until the threat is incapacitated. Contrary to human instinct, if a gunman enters a room with 20 people, he is at the physical disadvantage, not you. Unfortunately, human instinct among the 20 people is usually "play dead" or "beg" or to simply freeze and cower. The reaction instead should be let's get this asshole, and charge him. A gun is not a superweapon. It's very easy for 20 people to overpower one gunman, and while it seems like suicide to run at a gun, 20 people running at a workplace or school gunman have a much higher chance of surviving a close quarters fight with him than they do if they simply sit there or drop down.

Sad that this is even relevant these days, but it's important to know.