There are 2 main reasons an efficiency bar can appear:
1. Overworking Edit
When staff has worked for a long time, their efficiency will go down. A bar will appear and it will start to shrink the longer your staff are working without a vacation. You can give your staff members a vacation by clicking on them and clicking "Send On Vacation...". If the efficiency bar was low enough prior to the staff member being sent on vacation, as soon as the efficiency bar is filled, they will return from their vacation. If their bar was too high, they will stay out on vacation until the minimum amount of weeks of vacation (4 weeks).
2. New staff Edit
When a new staff member joins your company, their efficiency(bar) will be on it's lowest point. You can fill their bar by either giving them work to do, or giving them the 'staff welcoming traning'. All your other staff members will also have a big efficiency penalty, although they do not have a 'staff welcoming training' available.