Can I replace "entirely" with "all"?

What does "miss" mean here?

"Miss" here refers to feeling sad or regretful because an occurrence is not happening anymore, or you are not seeing someone anymore. Here, he is saying that he feels sad and regretful because he and his friend don't get together often anymore. Ex: I miss my mom, I haven't seen her in weeks. Ex: She misses going to the bakery that closed last month.

What's "hone" mean and in what situations can I use this word?

"To hone" means to literally make something sharper, usually the blade of a knife. Metaphorically, one can "hone" their skills, meaning to improve them. This is the most common use of the verb today. In most cases, it is used in the expression "to hone one's skills", meaning to improve or get better at something. Ex: I wish I was better at guitar. I should hone my skills. Ex: My brother spent a lot of time honing his driving skills, he's a great driver now!

What's "reservation" mean here?

Here, "reservation" refers to a doubt or reason for not agreeing with something completely, like a business deal here. It can be understood as hesitation or doubtfulness about something. Ex: Why do you have reservations about this situation? Do you feel nervous about something? Ex: She had reservations about moving halfway across the country for school.

Can I skip "down" here? Does it still make a sense without "down"?

Good question! Yes, you can omit "down" here. "Down" in this context is a preposition and makes the term "descend down" become a phrasal verb. However, to "descend" already implies a downward movement, and so the preposition "down" is not necessary here but can just be used to add emphasis.

What does "I dare you" mean? When can I use this expression?

"I dare you" is a phrase used to challenge another person to do something scary or difficult that they probably don't want to do. It is most commonly said by children. Ex: I dare you to go on that big roller coaster! Ex: I dare you to talk to the boy you like.