Countries around the world recognized and outlined all of the human rights that individuals have simply for being human, and they organized them into a body of international law called the Universal Declaration of Human Rights (UDHR), as well as the UN Charter, and various international treaties. The United Nations then decided, in order to make sure that countries, including the United States, are actually upholding these human rights, that they would be subject to a review of their human rights record every five years. This is what the United Nations Universal Periodic Review (UN-UPR) is.
 
The UPR is a unique mechanism of the Human Rights Council (HRC) aimed at improving the human rights situation on the ground in each of the countries that are members of the United Nations (called UN Member States). During this process, the US government typically submits a report to the UN explaining everything they are doing to uphold human rights in the US. Then, US Civil Society (grassroots activists, advocates, non-profit organizations, etc.), submit “stakeholder” reports which typically explains what is actually happening on the ground with various human rights (ex. immigration, gentrification, environmental justice, LGBTQIA rights, etc.). This typically contradicts what the U.S. governments says that it is doing. Then the UPR Working Group at the UN, and other countries can question the U.S. on issues outlined in the stakeholder reports. 
 
The result of each review is reflected in the Final Report of the Working Group, which lists the recommendations the U.S. [State under review (SuR)] will have to implement before the next review.
 
For more detailed information about the UPR, visit the United Nations Human Rights Council’s website.