This is the 678th most frequent Japanese word.
再
The Japanese word "再" means "again" or "re-."
Here, '再' is used as a prefix in '再利用' (さいりよう), meaning 'reuse'.
再利用が環境に優しいです。
Reusing is environmentally friendly.
In this example, '再' is a prefix in '再検討' (さいけんとう), meaning 'reexamination' or 'review'.
計画を再検討する必要があります。
It is necessary to reexamine the plan.
In this sentence, '再' (ふたたび) is used as an adverb meaning 'again'.