An adaptogen is a popular but loosely defined marketing term for herbs and supplements said to help the body "adapt to" or "resist" stress. It is not a formal, regulated scientific category, and the evidence behind most products labelled this way is limited or mixed.
The most-studied example is ashwagandha, which has some short-term trial evidence for perceived stress and cortisol — read the honest detail in ashwagandha for cortisol. As with any supplement, "adaptogens" are not cures, are unregulated, and can interact with medicines, so treat bold claims with caution and check with a clinician.