An inaccessible hide is any hide where your dog can not get their nose on (or reasonably close) to source.  As dogs source these hides, they will try to get close to source by going side to side, this is called bracketing and it is important to note. Inaccessible hides are very important to begin to practice because your dog needs to learn how to deal with a bit of ambiguity when it comes to sourcing.  If you only practice accessibles with your dog, they will not be able to narrow down difficult hides and will just keep bracketing.  On the other hand, if you practice too many inaccessibles, your dog can get sloppy and feel that they do not need to get all the way to source, this can result in fringing.  I try to set no more than 1 inaccessible for every 10 hides. 
There are three types of inaccessibles.  
1. Reach:  a hide that is behind a desk or under a cabinet.  Somewhere where they just can’t fit deep enough into the space to get at it. 
2. Height: A hide that is too high for your dog to reach
3. Enclosed: A hide that is in a drawer or something similar. 
When practicing all these hides, it is important to start easy (crack your drawers, and keep your inaccessibles shallow), and slowly work your way towards deeper inaccessibles.  Finally, try to mark the hide before your dog’s final alert, remember that the yes zone will be large for these hides and your dog just needs to show interest in the area.  
A really nice search to set up is an accessible hide that looks like an inaccessible at first sniff.  This teaches your dog that if a hide seems to be inaccessible from one side, they always need to check the other side for an accessible hide.  This is a key lesson for you and your dog  to learn.