What is "coder"?

What does "step up" mean here?

ここでの「step up」とは、助けが必要なときに行動を起こす、率先して行動することを意味します。 「step up」とは、何かに一生懸命取り組み始めることも意味します。 例：He stepped it up at work to try and get the promotion.（彼は昇進できるように、仕事への取り組みを強化しました。） 例：After their father died, the oldest son really stepped up and took on more responsibilities to take care of the family.（父親が亡くなった後、一番上の息子が懸命に努力し、家族の世話をするためより多くの責任を引き受けました。）

What's "invest in" mean here?

To "invest in" something means to put one's time, effort, or resources into something to receive benefits from it later. For example, some people might invest money into stocks, other people might invest time in their family. Ex: I want to invest more time into my personal development. Ex: I will invest $1,000,000 into my friend's company.

What does "knock it off" mean? Also, as a compound word, what does the suffix "-off" mean?

"Knock it off" is an idiom! It's another way of saying "stop it" or "quit it". You say it when you are annoyed with what someone is saying or doing. In this context, "off" is not a suffix. "Off" as a suffix refers to a competition or contest! Ex: Knock it off, Steve. That's so annoying! Ex: Drake, knock it off! Your singing is too loud. Ex: Let's have a dance-off. => dance competition Ex: Are you ready for the cook-off? I'm gonna win.

There's a "colon" next to the word "it". In what situation is "colon" used?

When using a colon, a general rule is it has to be used after a complete sentence. A colon introduces lists, explanations, series, quotations, and to separate independent clauses. It can also be used to show emphasis like it was in this particular video. Here are some examples: Ex: I need you to grab a few things: milk, juice, eggs, and bread. Ex: She is in trouble: she needs help. Ex: I have to do this: for mom.

What does "kill someone to ~" mean?

The expression "killing me" or "kills me" in this case means it causes someone a lot of emotions of stress, anxiety, panic, or hurt. So "this is killing me to watch" in this case means that it is very difficult to watch because it causes anxiety. You can pretty much use this expression anytime you are worried or anxious about something that you don't know the result or outcome of yet. Ex: I haven't received an acceptance letter for the university I applied to yet. The anticipation is killing me! Ex: The suspense is killing me, just get to the point of your story! Ex: It kills me to think that she might be mad at me.