A TCG-only card is a card currently released in the TCG, but not the OCG.
Only a small fraction of cards are released to the TCG first, referred to as "TCG Premiere" or "World Premiere" cards. The vast majority of these debut in the TCG versions of Core Boosters (which currently introduce 16 each); other sources include 60 Card Sets, Mega-Tins, and tournament prizes (most notably the Match winners awarded at World Championships, as they are printed in English).
These cards are almost always imported to the OCG with the Extra Pack/World Premiere Pack series. As those sets release every September, the number of TCG-only cards fluctuates in a yearly cycle that peaks in August (with currently 68 Legal ones).