Testosterone is an androgen sex steroid hormone. A steroid hormone is one that is made from cholesterol. A sex hormone is one that regulates sexual changes in the body. An androgen is the male type of sex hormone.
The testicles of men make lots of testosterone. Ovaries and adrenal glands make testosterone too. But they make much less than the testicles do. So men have much higher testosterone levels than women.
Testosterone has two different kinds of effects. One effect is anabolic. This effect causes growth of muscle and bone. The other effect of testosterone is androgenic. These effects make the body look male. This includes the effects that boys have at puberty: growing a beard, making the penis and testicles bigger, and making the voice deeper.