Think of a buckler like a small round shield that knights would carry around in medieval times.
It's made of metal or wood and is usually about the size of a dinner plate. The knight would hold the buckler in one hand while they used their other hand to hold their sword or other weapon.
The buckler would be used to block and deflect attacks from the enemy. It's also great for quick jabs to stun the enemy or for pushing them away. So imagine playing a game where you hold a shield in one hand and a sword in the other, and you use the shield to protect yourself while attacking your opponent with your sword. That's kind of how a buckler was used!
They're not as commonly used these days since we have bigger, more protective shields and better armor. But bucklers were an important part of medieval warfare and remain an interesting part of history.