Modafinil is a smart drug, also known as a nootropic. It enhances your cognitive function in a variety of ways (more on that in a second). There are plenty of smart drugs, but modafinil stands in a class of its own for a few reasons:

It’s not a stimulant. Modafinil acts sort of like a stimulant, but it’s actually a eugeroic – a wakefulness-promoting agent. It doesn’t make you speedy or jittery like most classical stimulants do. Modafinil also doesn’t have a crash or withdrawal, the way many smart drugs do.
It’s not addictive.[1] In fact, modafinil can help people kick addictions.[2]
It has few to no side effects. Modafinil is very safe. I’ve hacked my brain with neurofeedback so much that I don’t see much of a benefit from modafinil now, but I took it every day for 10 years and saw no problems of any kind with it during that time.
It works. Really well. Have you ever seen the movie Limitless with Bradley Cooper? It’s based on modafinil. This stuff gives you superhuman mental processing, with few to no downsides.