Printed circuit board:
http://en.wikipedia.org/wiki/Printed_circuit_board
It's the board that all the components are on. When I say "high quality PCB", I mean they improved on the reference design board by doing things like using higher quality capacitors and chokes, more power phases, etc...
Basically it means that the underlying quality of the entire card is better - it's overengineered for durability, stability, and overclocking. This allows for safer, more stable overclocking than a reference card and it allows you to overvolt the card a bit without fear of exploding a VRM on the card.
Sometimes it's hard to tell which cards are fully custom or not, because a lot of companies release cards that are a reference PCB + custom cooler, which IMO are not really all that much better for overclocking than pure reference cards.