"Never Removed From Box
used for collectible items
that have not been taken out of their original boxes
since they left the factory
. This also implies
that the item is in mint
condition, although that's not explicitly
defined in the term.
In general however, especially on auction sites such as eBay, NRFB is probably not true. Any item that can be removed without ruining either the box or the item (unlikely...) most certainly has been, or at least it's safe to assume so. If an item is in good condition and has its original box it can safely be sold as NRFB, since it's virtually impossible to disprove that something has in fact never been removed from its box.
Items in factory sealed boxes are better sold as MISB (Mint In Sealed Box) since they then cannot possibly have been taken out of their boxes (and should therefore be worth more, even though some less serious sellers may reseal boxes to cheat buyers).