Mazars is an accounting firm that does taxes for Donald Trump and his companies. Congress subpoenaed them for the tax returns of President Trump and his companies as part of an investigation into whether the Ethics in Government Act was fulfilling its purpose.
Part of the law requires presidents to tell Congress certain details about their finances, and there was clear proof that the president had lied on his disclosure forms (he didn’t disclose his hush money payment to adult film actress Stephanie Clifford), calling the efficacy of the law into question. Congress subpoenaed Mazars to see if he had lied anywhere else, which would help inform Congress whether the law was working as intended.
The courts have long acknowledged that Congress has broad oversight powers that are essential for it to function. Acknowledging our system of checks and balances, they have made exceptions for the president when it comes to the official duties of the office. But regarding their personal lives, presidents have the same obligations as any other citizen. President Trump does not have the right to stop Congress from doing its job.