Ashwagandha (Withania somnifera), especially its root, is one of the most important herbs in Ayurveda, the Indian traditional medicine based on the principles of natural healing and holistic approach to alleviating health issues.
Ashwagandha in Sanskrit means "smell of the horse", which is perhaps a reference to its unique scent, as well as the ability to enhance strength.
It has been used literally for thousands of years (first mentioned around 9000 years ago in the Rig Veda) to increase energy, improve concentration, relieve stress and anxiety, respiratory ailments, and for many more uses.
Memory and concentration
Historically Ashwagandha was used to boost memory and improve concentration. Today, there is more and more concrete scientific evidence that Ashwagandha may indeed improve certain domains of cognition - memory, reaction time, attention, and concentration ability, as well as general performance in mental tasks.
That's why nowadays it is often, for obvious reasons, one of the main ingredients of various herb-based nootropics.
Stress and anxiety
When we think of nootropics or enhancing our mental performance in general, we are culturally inclined to think in terms of "boosting" certain properties. But Ashwagandha doesn't help cognition just by enhancing certain domains of cognition, but possibly through helping reduce stress and anxiety.
Stress and anxiety are not phenomena that just affect how we feel. They affect our cognitive abilities and thinking - everybody knows that during a prolonged period of stress and/or anxiety, the mind becomes clouded and dwells on the issues causing stress. This dwelling can turn into a negatively reinforcing feedback loop. The person does not have his/her intelligence decreased, but a large portion of the intellectual processing power is wasted on anxiety and stress, which manifests as lowered focus and concentration ability, memory worsening, etc.
Ashwagandha may indeed leverage its effect on stress and anxiety to increase its cognitive enhancing abilities mentioned in the previous paragraph.
Testosterone and fertility in men
Ashwagandha root, and supplements in general, may have very powerful effects on testosterone levels and reproductive health of men. This is another positive benefit of this plant that has been known for millennia, but only recently modern studies started pinpointing the exact mechanisms.
Muscle mass and strength
As its Sanskrit meaning implies, Ashwagandha may indeed boost muscle mass and strength, making it, together with its testosterone boosting properties, a unique supplement for men.
Other potential benefits
Ashwagandha, like other herbal supplements, contains hundreds of molecules that may have a myriad of positive benefits on inflammation, cholesterol levels, depression, cortisol levels, and blood sugar levels.