What is MKR USD?

Could you possibly explain what exactly MKR USD refers to? I've heard about cryptocurrencies but I'm not entirely clear on this particular pairing. Is it a type of exchange rate? Or is it some sort of token or coin? Could you also elaborate on its purpose and how it fits into the larger cryptocurrency ecosystem? Additionally, how does the value of MKR USD fluctuate, and what factors influence its price movements? I'm quite curious to learn more about it.