This is the 378th most frequent Chinese word.
代
"代" means "generation," "era," "to substitute," or "on behalf of."
Here, '代' is found in '古代', meaning 'ancient times'. It denotes a specific period of time in history.
那个古代的皇帝非常有名。
That emperor from ancient times was very famous.
'代' in '现代' means 'modern'. It is used to refer to the present or contemporary era.
这种饮料适合现代年轻人。
This drink suits modern young people.
In this sentence, '代' is used in '代替', meaning 'to take the place of or substitute'. It indicates acting on behalf of someone else.