What is 「~in'」 means?
Eg:rockin'
I always saw this kind of words in text and article.
Is it the same meaning as「~ing」?
If so, why people use this instead of using 「~ing」?

查看翻譯

-in' is sometimes used casually/informally instead of -ing.

In spoken English, sometimes -ing endings sound more like -in endings (depending on the word and the person's accent), so it is sometimes written that way to emphasize the -in pronunciation. In a fictional story, -in' may be used to indicate that's how the character pronounces the word.

The apostrophe ( ' ) is used to indicate that letter(s) have been intentionally omitted. In this case, the g has been omitted.