GPM's right, it's a bunch of Trainer cards that all have the Holon tag on them, being Mentor, Farmer, Lass, Tranciever and possibly Researchers, depending on your deck.
The idea behind the Holon Engine is that you can utilize Holon Transceiver to pick out any Holon Supporter you need from your deck, which lets you use them to their maximum potential. A typical Holon Engine includes 4 Holon Transceiver, and up to 3 (usually 2) of various Holon Supporters, depending on the deck's needs.
Holon Transceiver can fetch Holon Supporters from the discard pile as well, which makes the Engine run that much better.