An aklys is a fancy way to describe a really old type of weapon that was used a long time ago. Imagine a big stick or club that has something kind of like a hook on the end of it. The hook part was used to grab onto things - like an enemy's shield - and pull it away so they couldn't protect themselves.
It's kind of like playing tug-of-war, but instead of rope, you're using a big stick with a hook on the end. People who used an aklys were able to surprise their enemies and have a better chance of winning a fight. It's not a common weapon anymore, but it's still cool to learn about!