Herbal tea was a variety of tea that was made from anything other than the leaves of a tea bush.
Neelix stocked several types of tea aboard the USS Voyager, including Terran and more exotic varieties. He himself favored herbal tea. (VOY: "Unforgettable")
The Star Trek Encyclopedia (4th ed., vol. 1, p. 334) described this tea as being made from aromatic plants. An example of this plant would be the chamomile.