It’s some kind of herb.
Healthy male subjects (aged 18-50), with limited training experience, were randomly assigned to receive a daily dose of either 300mg of ashwagandha root extract or placebo for 8-weeks. During those 8-weeks, subjects were also put on a 3-day/week resistance-training program. [Total testosterone levels] in the ashwagandha group increased, on average, by 15.25%.
In placebo, the increase was only 2.6%. But the really interesting results came in the form of strength gains. The ashwagandha group increased their bench press 1-RM by an astounding 138%. In placebo, the increase was 84%. Leg extension 1-RM for the ashwagandha group increased by 52%, compared to placebo whose increase was 38%.