Okay kiddo, so do you know what wolves are? In a pack of wolves, there's typically one alpha wolf - he's the leader. The other wolves in the pack all have different levels of importance too, but the ones who aren't the leader or the top dogs are often called beta wolves.
Now, humans aren't wolves, but sometimes people use the term "beta male" to describe a guy who they think isn't very confident or strong. They might say he's not as tough or successful as other men. Sometimes it's used as an insult, but it's not a nice thing to say about someone because it can hurt their feelings and make them feel bad about themselves.
It's important to remember that every person is unique and special and everyone has their own strengths and weaknesses. So, we shouldn't label people or put them in boxes based on a silly idea of what a "beta male" is supposed to be like.