When combined, the characters create a vivid, literal picture: “Strength (力) does not (不) follow (从) the heart/mind (心).” Your physical power or ability cannot follow the commands of your intentions.
力不从心 touches upon a deep-seated cultural value in China: the expectation of diligence, contribution, and fulfilling one's responsibilities. There is often a strong desire to try one's best (尽力, jìnlì) for the sake of family, a company, or a collective goal. This idiom gives voice to the painful and sympathetic situation where that desire exists, but the physical or practical means do not. In Western culture, a similar concept might be “burnout” or feeling “overextended.” However, there's a key difference. “Burnout” often carries a connotation of a systemic failure—poor work-life balance, a toxic job environment—and the solution is often seen as changing the external circumstances. 力不从心 is more internal and personal. It speaks to a fundamental mismatch between one's will and one's capacity, which can be due to age, illness, or overwhelming circumstances. It often evokes a sense of sympathy and resignation rather than a call to action. It acknowledges a personal limit with a sigh, not a protest.
This idiom is common in both spoken and written Chinese. It's used to express personal feelings or to describe someone else's situation with empathy.