Rhodiola rosea is a herb in the rhodiola genera (Crassulaceae family) which has traditional usage as an anti-fatigue agent and adaptogen compound. Its main chemicals that are thought to be responsible for its effects are rosavin and salidroside.
Rhodiola appears to be able to significantly reduce the fatigue and 'burnout' that come from stress and anxiety; numerous trials suggest meaningful effects, particularly in people with stress and anxiety issues. This may extend to exercise, since there's some evidence that acute supplementation before exercise can reduce fatigue, but more research is needed before we can be confident in that. Furthermore, it seems to have an effect on reducing stress and anxiety themselves, and may improve mood generally. Whether or not this is secondary to a reduction in fatigue is hard to say. As can be expected from a reduction in fatigue, research suggests that it can improve various measures of cognitive function, assuming that fatigue is reduced.
Other potential uses of rhodiola include preliminary evidence that it is highly neuroprotective against toxins (requires more evidence) and that ingestion of rhodiola or its active component can reduce stress-induced binge eating in female rats. In the brain, Rhodiola appears to be highly serotonergic (increases serotonin) and reduces corticosteroids; the inhibition of monoamine oxidases (MAOs) commonly attributed to rhodiola may not be relevant following oral ingestion of rhodiola, however. Rhodiola may also promote longevity, with preliminary (non-mammalian) evidence suggesting up to a 20% increase in lifespan secondary to mechanisms that are indepedent of caloric restriction. Although very promising, mammalian evidence is needed to confirm.
It's likely to be safe in the short to mid term (months to a year) when taken in normal doses. As with pharmacologically potent herbs in general, it may have notable drug interactions.
One study has found that some commercial Rhodiola products may be diluted or otherwise adulterated.