Refresher means to renew or reinvigorate. Refresher training should be carried out for existing staff, some people say annually at a minimum, but I prefer a little and often. Refresher training can take the form of a big 1/2 day training course (and sometimes it should) or it could be a memo, some new posters put up, a debate, workshop, shop floor talk, visit to a customer or supplier etc. It should really focus on areas of weakness or area for improvement such as complaints audit results etc. In my opinion for training to stick in the employees minds it should be short, sharp, innovative and even entertaining. Not an easy task with this type of training.
Shameless plug. 
The IFSQN (formerly Saferpak) developed an innovative range of hygiene training materials for packaging some year's back and they are still very popular for induction and refresher training in packaging companies all over the world.
You can find them in our store >>
Regards,
Simon