"Difficult to describe", but there are individuals who consider this taste another of the big ones. In fact, it's a bit contradictory since the other tastes alone are very recognizable. However something looks clear: it should produce signals so there are specific receptors that create them. Probably, there are more of this kind, capable of releasing a perception.
What makes a taste full? In theory, with more signals, and in the right quantities, the brain must have more nuances and a sense of completion. With less, perhaps a lifeless experience.
Unami, the mysterious taste, appears with the use of sodium glutamate, very usually obtained from natural sources. At least that is what it looks like according to different sources. Some do not believe in this new taste, and consider it a chemical byproduct of different combinations as the tonge a very complex chain of receptors working together thanks to a sophisticated mechanism with too many factors to consider; more than five or six.