"secret" seems like not countable. But how it has an article "a" in front of it?

Europe and America were raided by infamous pirates like Barbary or Caribbean pirates. Nevertheless, pirates are a popular source in literature and media. Why is that?

That's a good observation! The origins of the Western fascination with pirates is quite interesting. Around the time that newspapers became popular, pirates were very active around Europe. Since the 18th century, they have been some of the most idealized, romanticized figures in history. Although they were reviled as outlaws and criminals, there was also a fascination surrounding them because they seemed adventurous, free, and sometimes even heroic. The representation of pirates in famous American movies like Pirates of the Caribbean has boosted their popularity and allowed them to become icons in popular media.

Why would the speaker say "if he chooses?" Is it common for monarchs to change their names?

Yes, it is common! When someone in line to the British throne takes over the throne, they have the chance to change their name to whatever they want. For example, King George VI was Albert Frederick Arthur George. So if he wanted to, or "if he chooses," he could change his name. Queen Elizabeth the 2nd chose to keep her own name, as did her son, Princes Charles, now known as King Charles the 3rd.

What's "walk the walk, talk the talk" mean?

To "talk the talk and walk the walk" means to say what you're going to do, and actually do it. It's a commonly used idiom, usually used to indicate if someone does or doesn't do what they say or preach that they do. Ex: Jim talks the talk about recycling, but he doesn't walk the walk. => doesn't actually recycle Ex: She talks the talk and even walks the walk with her values.

Can you explain me about the expression "urge someone to ~"?

"Urge someone" means to strongly encourage or attempt to convince another to do or believe in something; it's more about getting another to do something quickly and conveying the importance of that action. Here are a few examples. Ex: I urge you to take your medicine! It'll keep you healthy. Ex: The government is urging everyone to stay home if they are sick. Ex: She was urging Jim to get his car fixed.

What's "watch their backs" mean?

To "watch one's back" means to protect or be wary of unexpected dangers. In essence, it means to look out for oneself and to be cautious. Ex: You watch my back and I'll watch yours. Ex: Even though she was careful to watch her back, she didn't anticipate her friend's betrayal.