In short
Curcumin is the primary bioactive polyphenol in turmeric (Curcuma longa), studied for anti-inflammatory and antioxidant properties, whose clinical usefulness is heavily constrained by very poor natural oral bioavailability.
In cell and animal studies, curcumin has demonstrated anti-inflammatory activity through multiple pathways, including inhibition of NF-κB, a master transcription factor that drives expression of many inflammatory genes. The challenge translating this to human clinical benefit is pharmacokinetic: unmodified curcumin is poorly absorbed from the gut, rapidly metabolized by the liver and gut wall, and rapidly eliminated, so that plasma concentrations after an oral dose of standard curcumin powder are very low relative to the concentrations used to produce effects in cell-culture studies.
This bioavailability problem is exactly why formulation matters more for curcumin than for many other supplements: products combining curcumin with piperine (black pepper extract, which inhibits a liver enzyme that would otherwise clear curcumin quickly), or using lipid-based, nanoparticle, or phospholipid-complex formulations, have shown substantially higher blood levels in human pharmacokinetic studies than standard curcumin powder, and most of the positive human trial results have used one of these enhanced-absorption formulations rather than plain turmeric extract.
With that caveat, a number of human randomized trials using enhanced-bioavailability curcumin formulations have shown reductions in markers of osteoarthritis pain (with effect sizes in some trials comparable to over-the-counter NSAIDs) and modest improvements in some inflammatory and lipid markers. Evidence for curcumin producing a broader anti-aging or lifespan benefit in humans, as opposed to these specific inflammatory and joint-pain outcomes, is not established.