Imagine you are sent back in time, to Western Europe during the "Middle Ages".
You might think:
with my modern knowledge, I will be able to invent marvelous things that make me the most rich, wise and powerful person around!
But what would you actually "invent"?
Most things that spring to mind could not be understood nor built from scratch by a single person. We rely on so many underlying inventions and so much infrastructure; supply chains, modern manufacturing techniques, material science, mass produced components.
You can't start making a cool new iPhone app if there is no iPhone, no microchip, etc.
But the compound bow is one example of a thing that could have been invented a long time ago, but it wasn't.
So if you learn a bit about compound bows, and how they work, you will be ready to become the famous inventor for the king, if you ever fall back in time!
Study this picture well:
Image from Lacaster Archery Supply
Unfortunately, at this point, the King will kill you to stop you sharing your knowledge with others.
So, make sure you also learn how to choose a fast horse and ride it well.
If you go far enough back in time, the stirrup is another amazing invention that by some accounts caused the Middle Ages.