This is the 505th most frequent Japanese word.
充実
"充実" means "fulfillment" or "being well-rounded and complete."
In this sentence, '充実' is used to describe that the content of the book is thorough and satisfying.
この本は内容が充実している。
This book is rich in content.
Here, '充実' refers to making his leisure time more fulfilling and meaningful.
彼は余暇を充実させるために趣味を始めた。
He started a hobby to enrich his leisure time.
In this example, '充実' signifies living a fulfilling and satisfying daily life.