Sesamin is the most prominent lignan compound found in sesame seeds, one of the two highest sources of lignans in the human diet (the other being flax). Sesamin is catered to be a nutritional supplement that confers antioxidant and antiinflammatory effects (if touting its health properties) or possibly being an estrogen receptor modulator and fat burner (if targeting atheltes or persons wishing to lose weight).
Sesamin has a few mechanisms, and when looking at it holistically it can be summed up as a fatty acid metabolism modifier. It appears to inhibit an enzyme known as delta-5-desaturase (Δ5-desaturase) which is a rate-limiting enzyme in fatty acid metabolism; inhibiting this enzyme results in lower levels of both eicosapentaenoic acid (EPA, one of the two fish oil fatty acids) as well as arachidonic acid, and this mechanism appears to be relevant following oral ingestion. The other main mechanism is inhibiting a process known as Tocopherol-ω-hydroxylation, which is the rate limiting step in the metabolism of Vitamin E; by inhibiting this enzyme, sesamin causes a relative increase of vitamin E in the body but particularly those of the gamma subset (γ-tocopherol and γ-tocotrienol) and this mechanism has also been confirmed to be active following oral ingestion.
There are some other mechanisms in place which seem promising (protection against Parkinson's Disease as well as promotion of bone mass) but most of the mechanisms including estrogen receptor modulation, fat burning from the liver, and activation of the antioxidant response element (ARE) have not been confirmed in humans and have their reasons to suspect that they do not occur; this includes either a concentration that is too high to matter for oral supplementation, or in the case of fat burning it being a process that seems to be exclusive to rats.
In the end, sesamin serves a pretty interesting role as having the potential to augment γ-tocopherol and γ-tocotrienol metabolism by preventing their degradation; increasing the levels of these vitamin E vitamers has a lot of therapeutic benefits in and of itself, and since they are quite expensive to purchase as supplements then sesamin could be a cheap workaround or something used to 'cut' the vitamin E.