What does 1 Corinthians 12:26 mean?

Body parts don't literally have individual emotional experiences. And, as Paul suggests in this verse, it's not entirely possible for one body part to be "happy" when some other part is deeply suffering. People don't really notice their hands are feeling fine when their feet are in agony. Bodies are single organisms. If one part of the body suffers, the rest of the body suffers along with it. The hand will do whatever it can to relieve the pain in the foot and vice versa.

Once again, Paul's analogy demonstrates a deep level of sophistication. A common mantra in workplace safety is "lift with your legs, not with your back." The point is that if the body part designed for a task—the legs—does not fulfill that role, some other part—the back—must try. By definition, that other part is not as well-suited for the task. That leads to over-exertion, stress, and injury. One way to keep the back from being strained is for the legs to fulfill their role. When each body part cooperates to meet the needs of their unique design, the body acts with maximum comfort and efficiency.

The church never functions more correctly, according to God's design, than when Christians do the same thing for each other. When we are willing to be moved by the experiences of our brothers and sisters in Christ, to feel pain and joy at the pain and joy of others, to take on the roles we have been given by God, we move closer and closer to being that single Christ-like organism. It is who God intends for us to be.