Theaflavins are a group of molecules that are created from green tea catechins when heat treatment damages the catechin molecule and they begin to resynthesize in various configurations; Theaflavins are the product of two catechin molecules binding together on the C ring, and similar to catechins can come in various gallated forms (whether there is gallic acid molecules bound to them or not).
Currently, Theaflavins appear to have really poor oral bioavailability and absorption. For systemic effects after absorption, it is likely that the bioactivity of Theaflavins (which is present) is due to a metabolite that is not Theaflavins. For most evidence outside of the gastrointestinal tract (mouth to anus), the molecule of Theaflavins per se may not be relevant unless below the 1 nanomolar range.
For effects within the gastrointestinal tract, Theaflavins appear to have promising anti-ulceration and oral cavity health properties that should be active at the concentrations found in black tea. Either black tea or supplementation also appears to reduce the absorption of fatty acids, cholesterol, and starches from the intestinal tract which can reduce nutrient absorption quantities (and secondary to inhibiting starch absorption, there may be some probiotic effects similar to dietary fiber).