What is an example of theta?

Could you possibly enlighten me on the concept of theta? Could you provide an illustrative example to clarify its application or manifestation? It would be helpful if you could keep your explanation concise yet comprehensive, ideally within a word count that doesn't exceed three hundred. I'm particularly interested in understanding how theta manifests in practical scenarios, perhaps within the realm of finance or cryptocurrency. Thank you for your assistance in clarifying this topic for me.