it means that people are practiced in carrying out an emergency procedure (opening fire escapes, clearing passages, closing fire doors, unlocking through doors, closing kitchens down (making safe) etc so that when they have to do it in an emergency, they are well versed in what to do. Similar principle to aircraft pilots practicing on flight simulators. It means that any issues such as 'where are the keys for the joining doors between hospitality and gen admission' can be sorted before they are REALLY needed
I now work in the Health and Safety world, it really makes sense to test a safety system before it is needed