I was watching a couple of miniature painting tutorials and both times the narrator referred to "sealing" the model. I'm not quite sure what he meant by this. What does sealing a model refer to?
Sealing a model means applying a coat of a transparent and durable protective layer after painting. Normally this is a clear varnish. After leaving your painted model to thoroughly dry, you brush or spray it on. This helps prevent chipping and flaking of your paint job during subsequent transportation and gaming.
You have a basic choice of gloss or matte finish, depending on whether you like your models more reflective or not.
The main downside of sealing is that it makes applying further paint changes or touch ups very difficult. So be sure you're satisfied with your paint job before you seal!