What is a knight?
The simple answer is: a soldier in the Middle Ages, who fought for his lord, who was his boss.
A knight was an important soldier with a high rank. He was usually fighting on horseback and often commanded other soldiers in a battle.
During the Middle Ages, knighthood was considered a class of lower nobility, which means he was more important than most common people, but much lower in importance than the king, dukes, counts, barons and other nobel people.
Here's an image showing a knight on his horse, ready for battle.