To reabsorb means to absorb something again, taking it back in after it was released or separated. Your kidneys reabsorb water and nutrients from the liquid passing through them, pulling useful substances back into your bloodstream instead of letting them leave your body as waste. Without this process, you'd lose essential minerals and become dangerously dehydrated.

When a sponge absorbs water and you squeeze it out, then let it soak up that same water again, it reabsorbs it.

A broken bone reabsorbs some of its own material as part of healing, with your body breaking down damaged bone tissue before rebuilding it stronger. Scientists studying climate change examine how oceans reabsorb carbon dioxide from the atmosphere. Even organizations can reabsorb ideas or methods they had previously abandoned, bringing back old approaches that turn out to be valuable.

The word always suggests a cycle: something goes out or separates, then gets taken back in. It's different from simply absorbing something for the first time.