The characters combine to mean “the joy of the natural family order.” The happiness described is not random but comes specifically from the fulfillment and harmony of these fundamental family roles.
“天伦之乐” is a cornerstone of Chinese cultural values, deeply intertwined with Confucian philosophy. Confucianism emphasizes a structured society built upon key relationships (五伦, wǔlún), with the family being the most critical unit. The happiness of “天伦之乐” arises when these relationships are in harmony—when children are respectful (孝顺, xiàoshùn), parents are loving, and elders are cared for. A helpful comparison for a Western learner is to contrast it with the concept of “quality time.” In the West, “quality time” often implies a planned activity, focused attention, and active engagement (e.g., “Let's go to the park and play catch for some quality time”). “天伦之乐,” however, can be much more passive and ambient. It can be found in the simple act of existing in the same space—grandma knitting in a chair while her grandson does his homework nearby. The joy is in the shared presence and the unspoken understanding of familial bonds, not necessarily in the event itself. It's a state of being, a feeling of completeness, rather than a scheduled activity.
While it is a classical and somewhat formal idiom (chengyu), “天伦之乐” is widely understood and cherished in modern China.