How much is one arb?

Excuse me, could you clarify for me what exactly an 'arb' is and how much it is worth? I'm not familiar with the term in the context of cryptocurrency or finance. Is it a specific unit of currency, a type of token, or something else entirely? And if it does have a monetary value, what determines that value and how does it fluctuate over time? I'd appreciate any information you can provide to help me understand this term better.