Clarity means clearness. Clean water running down a mountain has clarity. So does a lovely singing voice: it's clear and pure.
If you bring clarity to a situation, you help people see what really happened by clearing up misunderstandings or giving explanations. Sometimes people talk about having moments of clarity when suddenly everything about a particular situation or even about life itself comes into focus. People also talk about good writing having clarity when it is precise, clean, and easy to understand.