Relatability is a quality of being connected — either literally, like the relatability of your good mood and the beautiful sunny weather, or emotionally, like the relatability of a TV character you identify with.
The relatability between things is a kind of cause and effect: there's a clear relatability between how much fried dough you eat at the fair and how your stomach feels later that day. It's even more common to use this word for a kind of sympathy, understanding, or connection between people: "I know the other mayoral candidate has more political experience, but her relatability makes me want to vote for her. She seems like a real person, you know?"