This term was first introduced with DMPP-09 ARCADIA OVERDRIVE to simplify effect text for cards that add shields into your shield zone as well as for cards that can put creatures into their owner's shields as a form of removal.
Rather than including the text of "into its owner's shields face down", this text is omitted and assumed.
Examples of this wording change include;
Shield Addition[]
■ Add the top card of your deck to your shields.
becomes
■ Shieldify the top card of your deck.
Shield Feed[]
■ Add one of your opponent's non-evolution creatures to it's owner's shield zone.
becomes
■ Shieldify one of your opponent's non-evolution creatures.
Related Categories[]
For cards that give add additional shields, see Shield Addition.
For cards that give put creatures into their owners shield zone, see Shield Feed.