What does "reincarnation" actually mean? Does it have anything to do with karma?
The eternal soul passes from boyhood, to youth, to old age, and then eventually into another body at the time of death.
The Vedic literature tells us that we souls can inhabit any of millions of forms of life, including aquatics, plants, insects, reptiles, birds, animals, and human beings. At the time of death, we leave one body and enter a new one. That is called reincarnation.
We can observe that we change from one body to another in our own lifetime. Your body at birth is completely different from your adult body. Yet throughout these changes, you—the conscious self—remain the same. Similarly, the conscious self remains the same from one body to the next in the cycle of reincarnation.
Our present body is the result of a long series of actions and reactions in previous lives. The law that governs this is known as karma: every action has a reaction. Our previous actions have produced our present body, and our current actions will determine our next body.
Krishna, through the agency of material nature, provides all these bodies for us in response to our desire to enjoy in the material world. Yet all species of life also share in the miseries of the material world. So for us eternal souls, moving from body to body means experiencing every imaginable kind of suffering. Only in the human form can we free ourselves from the vicious cycle of reincarnation—by reestablishing our eternal, loving relationship with Krishna.