In a Nutshell: `充实` describes a state of being full, not in a physical sense, but in a qualitative one. It’s the feeling you get when your time is filled with meaningful, productive, or enriching activities, leading to a sense of satisfaction and personal growth. It's the opposite of feeling empty, bored, or that you've wasted your time. Whether describing a single day, a whole life, or the content of a book, `充实` implies substance, value, and purpose.