Okay kiddo, so you know how sometimes people do really good things or help lots of people and they get a special award or medal? Well, a knight bachelor is kind of like that, but instead of an award or medal they get called a knight and get to use the title "Sir" before their name.
Basically, a knight bachelor is a man who has been recognized by the king or queen of England for doing something really good or important. It could be something like helping out in the community, doing great work in their job, or even fighting for their country in a war.
When someone is made a knight bachelor, they get a special ceremony where the king or queen taps them on the shoulders with a sword and says some special words. After that, they get to call themselves "Sir" and are allowed to wear a special badge on their clothes.
While being a knight bachelor is a big honour, it doesn't mean they get to live in a castle or have a suit of armour like you might see in movies. They just get to use the title "Sir" and are recognized as having done something really great.