What does it mean that the younger brother is the best copywriter?

It can be understood as: The younger brother is the best and most outstanding advertising copywriter or editor.

First of all, "brother" is a term of endearment, indicating that the speaker has a younger brother.

Therefore, the sentence "My younger brother is the best copywriter" can be understood as: the speaker recognizes and appreciates his younger brother's advertising copywriting very much, and thinks that his copywriting is the best and most attractive. This may be based on the brother's actual experience and performance in the advertising industry, or it may be an emotional expression of the speaker towards him.

In addition, this sentence can also be interpreted as a kind of encouragement and support, that is, the speaker believes that his younger brother can achieve greater success in this industry. This kind of encouragement and support can help the younger brother to be more confident and use his talents well at work.

In short, the sentence "My younger brother is the best copywriter" is a kind of praise and recognition, expressing the speaker's appreciation and love for his younger brother's performance and talents in the advertising industry.