What does "sidetracked" mean? How to use the word?

What does "bum" mean? and what expression can I use instead of "bum"?

Good question! Be careful, because "bum" and "bummed" mean two different things. "Bum" is British English for "butt", and "bummed" means "disappointed". You can simply replace "bummed" with "disappointed", they mean the same thing. Ex: I was really bummed I couldn't go to the amusement park yesterday because it was raining.

If the wife has the higher rank in the monarch, just like Queen Victoria, in this case, does the male spouse changes their surname?

This is not the case. The British royal family, especially those close in relation to the monarch, doesn't have last names but titles instead. So, for example, if a woman were to marry a man in the royal family, she will adopt the feminine version of the title held by her husband. There is no adopting of last names within the royal family, regardless of whether the person with a higher "rank" is female or male. For example, when Diana Spencer married His Royal Highness (HRH) Prince of Wales, she did not adopt his surname (as he technically does not have one). Rather, she adopted the feminine version of his title, becoming Her Royal Highness (HRH) the Princess of Wales.

What's "worldly" mean when's it used?

Generally, "worldly" refers to material objects and ordinary life, usually as opposed to something spiritual, but here it's used in comparison to Hikaru Utada's career and art. They've been encouraged to focus on their music rather than ordinary things like opening a bank account. Ex: I like to meditate to shift my attention away from worldly things. It calms me. Ex: Worldly things aren't as important as peace and love. Ex: It's impossible to live without focusing on worldly things.

What's "rolling stone" mean?

Here, "rolling stone" refers to him moving constantly for work. It comes from the idiom "a rolling stone gathers no moss," which means that a person does not stay in one place to gain wealth or status. They avoid responsibilities and cares. It also has a musical reference to the rock band, The Rolling Stones, which links since he's singing about his musical career. Ex: He was a rolling stone for many years. Ex: I prefer to be a rolling stone. I don't want to live in one place.

What would be the meaning of "cooler" here?

In this video, "cooler" means awesome or interesting, seen in a positive light. Ex: He is so cool! Ex: Wow, this song is so cool! Ex: There's nothing that can be cooler than this.