This is the 7799th most frequent Chinese word.
闲
"闲" means "leisure," "free time," or "idle."
In this sentence, "闲" is used as an adjective describing someone's state of having free time or not being busy.
他很闲，常常出去散步。
He is very idle, often going out for walks.
Here, "闲" modifies "书" to imply that the book is not academic or professional but meant for leisure reading.
这本闲书非常有趣，适合休闲时阅读。
This casual book is very interesting, suitable for reading during leisure time.
"闲时" integrates "闲" to indicate a period when a person is free and not occupied.