In a floating raft system the roots sit in the water, so they need oxygen from the solution. An air pump and air stone supply it.
Why oxygen matters
Roots that lack oxygen can rot. Warm water holds less oxygen, which makes the problem worse.
Keep it running
Check the pump and air stone regularly. A stopped pump or a blocked stone can cause problems within a short time.
Keep the water cool
Keep the container out of direct sun and heat, so the solution holds more oxygen.