在美剧中老听见美国人说10-4？是啥意思？

10-4 means "OK," "understood," and "affirmative." Commonly known as CB radio lingo, originating from the need for brevity in radio transmission.
me--"Is anyone there?"
respondent--" 10-4"(to indicate presence).

me--"Let's roll."
respondent--"10-4"(to indicate agreement).

在urban dictionary里的解释 看懂了吧

最初是为了无线电发报时做的简写，意思就是：十分肯定，明白